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TABLE OF CONTENTS
AUDITED FINANCIAL STATEMENTS
TABLE OF CONTENTS
INDEX TO FINANCIAL STATEMENTS

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Filed Pursuant to Rule 497
Registration Statement No. 333-223483

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with and declared effective by the Securities and Exchange Commission. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell and are not soliciting offers to buy these securities in any state where such offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED APRIL 17, 2019

PRELIMINARY PROSPECTUS SUPPLEMENT
(to Prospectus dated April 27, 2018)

$            

        % Notes due

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          We are offering $            in aggregate principal amount of        % notes due            , which we refer to as the Notes. The Notes will mature on                  ,        . We will pay interest on the Notes on            and            of each year, beginning on                  ,        . We may redeem the Notes in whole or in part at any time or from time to time, at the redemption price set forth under the caption "Description of the Notes—Optional Redemption" in this prospectus supplement. In addition, holders of the Notes can require us to repurchase some or all of the Notes at a purchase price equal to 100% of their principal amount, plus accrued and unpaid interest to, but not including, the repurchase date upon the occurrence of a "Change of Control Repurchase Event" (as defined herein). The Notes will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof.

          The Notes will be our direct unsecured obligations and rank pari passu with our existing and future unsecured indebtedness but will rank senior to our future indebtedness that is expressly subordinated in right of payment to the Notes issued by Main Street Capital Corporation. See "Summary of the Offering—Ranking of Notes."

          We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $50 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $20 million.

          The LMM and Middle Market securities in which we invest generally would be rated below investment grade if they were rated by rating agencies. Below investment grade securities, which are often referred to as "junk," have predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal. They may also be difficult to value and are illiquid.

          Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company.

          We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company under the Investment Company Act of 1940, as amended.

          Investing in the Notes involves a high degree of risk and should be considered highly speculative. See "Supplementary Risk Factors" beginning on page S-10 of this prospectus supplement and "Risk Factors" beginning on page 15 of the accompanying prospectus to read about factors you should consider, including the risk of leverage, before investing in the Notes.

          This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in our Notes. Please read this prospectus supplement and the accompanying prospectus before investing and keep them for future reference. We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission, or SEC. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, 8th Floor, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus. The SEC also maintains a website at www.sec.gov that contains such information.

          Neither the Securities and Exchange Commission nor any state securities commission, nor any other regulatory body, has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

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          The public offering price set forth above does not include accrued interest, if any. Interest on the Notes will accrue from April     , 2019 and must be paid by the purchaser if the Notes are delivered after April     , 2019.

          THE NOTES ARE NOT DEPOSITS OR OTHER OBLIGATIONS OF A BANK AND ARE NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENT AGENCY.

          Delivery of the Notes in book-entry form only through The Depository Trust Company will be made on or about April     , 2019.

RBC Capital Markets

   

The date of this prospectus supplement is April     , 2019.

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Prospectus Supplement

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ABOUT THE PROSPECTUS

        This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of Notes and also adds to and updates information contained in the accompanying prospectus. The second part is the accompanying prospectus, which provides more information about the securities we may offer from time to time. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control.

        You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to buy, any Notes by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of our Notes. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

Forward-Looking Statements

        Information contained in this prospectus supplement and the accompanying prospectus may contain forward-looking statements, which can be identified by the use of forward-looking terminology such as "may," "will," "expect," "intend," "anticipate," "estimate," or "continue" or the negative thereof or other variations thereon or comparable terminology. The matters described in the sections titled "Supplementary Risk Factors" in this prospectus supplement and "Risk Factors" in the accompanying prospectus and certain other factors noted throughout this prospectus supplement and the accompanying prospectus constitute cautionary statements identifying important factors with respect to any such forward-looking statements, including certain risks and uncertainties that could cause actual results to differ materially from those in such forward-looking statements. We undertake no obligation to revise or update any forward-looking statements but advise you to consult any additional disclosures that we may make directly to you or through reports that we may file in the future with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K. We note that the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995 does not apply to statements made in this prospectus supplement or the accompanying prospectus.

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SUPPLEMENTARY RISK FACTORS

        Investing in the Notes involves a number of significant risks. In addition to the other information contained in this prospectus supplement and the accompanying prospectus, you should carefully consider the following supplementary risk factors together with the risk factors set forth in the accompanying prospectus before making an investment in the Notes. The risks set out below and in the accompanying prospectus are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the events described herein or in the accompanying prospectus occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, the market price, if any, of the Notes could decline, and you may lose part or all of your investment.

Risks Relating to Economic Conditions

Global capital markets could enter a period of severe disruption and instability. These conditions have historically affected and could again materially and adversely affect debt and equity capital markets in the United States, which could have a materially negative impact on our business, financial condition and results of operations.

        The U.S. and global capital markets have, from time to time, experienced periods of disruption characterized by the freezing of available credit, a lack of liquidity in the debt capital markets, significant losses in the principal value of investments, the re-pricing of credit risk in the broadly syndicated credit market, the failure of major financial institutions and general volatility in the financial markets. During these periods of disruption, general economic conditions deteriorated with material and adverse consequences for the broader financial and credit markets, and the availability of debt and equity capital for the market as a whole, and financial services firms in particular, was reduced significantly. These conditions may reoccur for a prolonged period of time or materially worsen in the future. In addition, continuing uncertainty arising from the United Kingdom's decision to leave the European Union (the so called "Brexit") could lead to further market disruptions and currency volatility, potentially weakening consumer, corporate and financial confidence and resulting in lower economic growth for companies that rely significantly on Europe for their business activities and revenues. We may in the future have difficulty accessing debt and equity capital markets, and a severe disruption in the global financial markets, deterioration in credit and financing conditions or uncertainty regarding U.S. government spending and deficit levels, Brexit or other global economic conditions could have a material adverse effect on our business, financial condition and results of operations.

Risks Relating to Our Business and Structure

We are dependent upon our key investment personnel for our future success.

        We depend on the members of our investment team, particularly Dwayne L. Hyzak, David L. Magdol, Vincent D. Foster, Curtis L. Hartman, K. Colton Braud, III, Nicholas T. Meserve, Samuel A. Cashiola and Watt R. Matthews, for the identification, review, final selection, structuring, closing and monitoring of our investments. These employees have significant investment expertise and relationships that we rely on to implement our business plan. Although we have entered into a non-compete agreement with Mr. Foster and non-compete arrangements with all of our executive officers and other key employees in connection with their restricted stock grants, we have no guarantee that he or any other employees will remain employed with us. If we lose the services of these individuals, we may not be able to operate our business as we expect, and our ability to compete could be harmed, which could cause our operating results to suffer.

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We, through the External Investment Manager, derive revenues from managing third party funds pursuant to management agreements that may be terminated pursuant to the terms of such agreements or requirements under the 1940 Act.

        The External Investment Manager earns management fees based on the assets of the funds under management and may earn incentive fees, or a carried interest, based on the performance of the funds managed, including HMS Income. The terms of fund investment management agreements generally give the manager of the fund and the fund itself the right to terminate the management agreement in certain circumstances. With respect to funds that are not exempt from regulation under the 1940 Act, the fund's investment management agreement must be approved annually by (a) such fund's board of directors or by the vote of a majority of such fund's stockholders and (b) the majority of the independent members of such fund's board of directors and, in certain cases, by its stockholders, as required by law. The funds' investment management agreements can also be terminated by the majority of such fund's stockholders. Termination of any such management agreements would reduce the fees we earn from the relevant funds through the External Investment Manager, which could have a material adverse effect on our results of operations. Currently, HMS Income, an investment company that has elected to be regulated as a business development company under the 1940 Act, is subject to these provisions of the 1940 Act.

Risks Relating to the Notes

The Notes will be unsecured and therefore will be effectively subordinated to any current or future secured indebtedness, including indebtedness under the Credit Facility.

        The Notes will not be secured by any of our assets or any of the assets of our subsidiaries and will rank equally in right of payment with all of our existing and future unsubordinated, unsecured indebtedness. As a result, the Notes are effectively subordinated to any secured indebtedness we or our subsidiaries have currently incurred and may incur in the future (or any indebtedness that is initially unsecured to which we subsequently grant security) to the extent of the value of the assets securing such indebtedness. In any liquidation, dissolution, bankruptcy or other similar proceeding, the holders of any of our existing or future secured indebtedness and the secured indebtedness of our subsidiaries may assert rights against the assets pledged to secure that indebtedness in order to receive full payment of their indebtedness before the assets may be used to pay other creditors, including the holders of the Notes. As of April 12, 2019, we had $293.0 million outstanding under the Credit Facility out of $705.0 million in commitments. The indebtedness under the Credit Facility is senior to the Notes to the extent of the value of the assets securing such indebtedness.

The Notes will be structurally subordinated to the indebtedness and other liabilities of our subsidiaries.

        The Notes are obligations exclusively of Main Street Capital Corporation and not of any of our subsidiaries. None of our subsidiaries is a guarantor of the Notes, and the Notes are not required to be guaranteed by any subsidiaries we may acquire or create in the future. In addition, several of our subsidiaries, specifically the Funds, maintain significant indebtedness and as a result the Notes are structurally subordinated to the indebtedness of these subsidiaries. For example, as of April 12, 2019, the Funds had collectively issued $321.8 million of the current regulatory maximum of $350.0 million of SBA-guaranteed debentures, which are included in our consolidated financial statements. The assets of such subsidiaries are not directly available to satisfy the claims of our creditors, including holders of the Notes. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources" in this prospectus supplement for more detail on the SBA-guaranteed debentures.

        Except to the extent we are a creditor with recognized claims against our subsidiaries, all claims of other creditors of our subsidiaries will have priority over our equity interests in such subsidiaries (and

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therefore the claims of our creditors, including holders of the Notes) with respect to the assets of such subsidiaries. Even if we are recognized as a creditor of one or more of our subsidiaries, our claims would still be effectively subordinated to any security interests in the assets of any such subsidiary and to any indebtedness or other liabilities of any such subsidiary senior to our claims. Consequently, the Notes will be structurally subordinated to all indebtedness, including the SBA-guaranteed debentures, and other liabilities of any of our subsidiaries and any subsidiaries that we may in the future acquire or establish. In addition, our subsidiaries may incur substantial additional indebtedness in the future, all of which would be structurally senior to the Notes.

There is no active trading market for the Notes. If an active trading market does not develop for the Notes you may not be able to sell them.

        The Notes are a new issue of debt securities for which there currently is no trading market. We do not intend to list the Notes on any securities exchange or for quotation of the Notes on any automated dealer quotation system. If the Notes are traded after their initial issuance, they may trade at a discount to their initial offering price depending on prevailing interest rates, the market for similar securities, our credit ratings, our financial condition or other relevant factors. The underwriters may discontinue any market-making in the Notes at any time at their sole discretion. Accordingly, we cannot assure you that a liquid trading market will develop for the Notes, that you will be able to sell your Notes at a particular time or that the price you receive when you sell will be favorable. To the extent an active trading market does not develop, the liquidity and trading price for the Notes may be harmed. Accordingly, you may be required to bear the financial risk of an investment in the Notes for an indefinite period of time.

A downgrade, suspension or withdrawal of the credit rating assigned by a rating agency to us or the Notes, if any, or change in the debt markets could cause the liquidity or market value of the Notes to decline significantly.

        Our credit ratings are an assessment by rating agencies of our ability to pay our debts when due. Consequently, real or anticipated changes in our credit ratings will generally affect the market value of the Notes. These credit ratings may not reflect the potential impact of risks relating to the structure or marketing of the Notes. Credit ratings are not a recommendation to buy, sell or hold any security, and may be revised or withdrawn at any time by the issuing organization in its sole discretion. Neither we nor any underwriter undertakes any obligation to maintain our credit ratings or to advise holders of Notes of any changes in our credit ratings. The Notes will be rated by Standard & Poor's Ratings Services, or S&P. There can be no assurance that our credit ratings will remain for any given period of time or that such credit ratings will not be lowered or withdrawn entirely by the rating agency if in their judgment future circumstances relating to the basis of the credit ratings, such as adverse changes in our company, so warrant. The conditions of the financial markets and prevailing interest rates have fluctuated in the past and are likely to fluctuate in the future, which could have an adverse effect on the market prices of the Notes.

The indenture under which the Notes will be issued contains limited protection for holders of the Notes.

        The indenture under which the Notes will be issued offers limited protection to holders of the Notes. The terms of the indenture and the Notes do not restrict our or any of our subsidiaries' ability to engage in, or otherwise be a party to, a variety of corporate transactions, circumstances or events that could have an adverse impact on your investment in the Notes. In particular, the terms of the indenture and the Notes will not place any restrictions on our or our subsidiaries' ability to:

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issue securities or otherwise incur additional indebtedness or other obligations, including (1) any indebtedness or other obligations that would be equal in right of payment to the Notes, (2) any indebtedness or other obligations that would be secured and therefore rank effectively senior in

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right of payment to the Notes to the extent of the values of the assets securing such debt, (3) indebtedness of ours that is guaranteed by one or more of our subsidiaries and which therefore is structurally senior to the Notes and (4) securities, indebtedness or obligations issued or incurred by our subsidiaries that would be senior to our equity interests in our subsidiaries and therefore rank structurally senior to the Notes with respect to the assets of our subsidiaries, in each case other than an incurrence of indebtedness or other obligation that would cause a violation of Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act or any successor provisions, but giving effect, in each case, to any exemptive relief granted to us by the SEC (currently, this provision generally prohibits us from making additional borrowings, including through the issuance of additional debt or the sale of additional debt securities, unless our asset coverage, as defined in the 1940 Act, equals at least 200% (or 150% if certain requirements are met) after such borrowings);

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pay dividends on, or purchase or redeem or make any payments in respect of, capital stock or other securities ranking junior in right of payment to the Notes, including subordinated indebtedness;



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create restrictions on the payment of dividends or other amounts to us from our subsidiaries.

        Furthermore, the terms of the indenture and the Notes do not protect holders of the Notes in the event that we experience changes (including significant adverse changes) in our financial condition, results of operations or credit ratings, if any, as they do not require that we or our subsidiaries adhere to any financial tests or ratios or specified levels of net worth, revenues, income, cash flow or liquidity.

        Our ability to recapitalize, incur additional debt and take a number of other actions that are not limited by the terms of the Notes may have important consequences for you as a holder of the Notes, including making it more difficult for us to satisfy our obligations with respect to the Notes or negatively affecting the trading value of the Notes.

        Other debt we issue or incur in the future could contain more protections for its holders than the indenture and the Notes, including additional covenants and events of default. The issuance or incurrence of any such debt with incremental protections could affect the market for and trading levels and prices of the Notes.

The optional redemption provision may materially adversely affect your return on the Notes.

        The Notes are redeemable in whole or in part upon certain conditions at any time or from time to time at our option. We may choose to redeem the Notes at times when prevailing interest rates are lower than the interest rate paid on the Notes. In this circumstance, you may not be able to reinvest the redemption proceeds in a comparable security at an effective interest rate as high as the Notes being redeemed.

We may not be able to repurchase the Notes upon a Change of Control Repurchase Event.

        We may not be able to repurchase the Notes upon a Change of Control Repurchase Event because we may not have sufficient funds. Upon a Change of Control Repurchase Event, holders of the Notes

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may require us to repurchase for cash some or all of the Notes at a repurchase price equal to 100% of the aggregate principal amount of the Notes being repurchased, plus accrued and unpaid interest to, but not including, the repurchase date. The terms of our Credit Facility provide that certain change of control events will constitute an event of default thereunder entitling the lenders to accelerate any indebtedness outstanding under our Credit Facility at that time and to terminate the Credit Facility. Our and our subsidiaries' future financing facilities may contain similar restrictions and provisions. Our failure to purchase such tendered Notes upon the occurrence of such Change of Control Repurchase Event would cause an event of default under the indenture governing the Notes and a cross-default under the agreements governing certain of our other indebtedness, which may result in the acceleration of such indebtedness requiring us to repay that indebtedness immediately. If a Change of Control Repurchase Event were to occur, we may not have sufficient funds to repay any such accelerated indebtedness. See "Description of the Notes—Offer to Repurchase Upon a Change of Control Repurchase Event" in this prospectus supplement for more information.

Our amount of debt outstanding will increase as a result of this offering, and if we default on our obligations to pay our other indebtedness, we may not be able to make payments on the Notes.

        As of April 12, 2019, we had approximately $974.8 million of principal indebtedness, including $293.0 million outstanding under the Credit Facility, $321.8 million outstanding from SBA-guaranteed debentures, $175.0 million of the 4.50% Notes due 2019 and $185.0 million of the 4.50% Notes due 2022 outstanding. Any default under the agreements governing our indebtedness, including a default under the Credit Facility, under the 4.50% Notes due 2019, under the 4.50% Notes due 2022 or under other indebtedness to which we may be a party that is not waived by the required lenders or debt holders, and the remedies sought by the holders of such indebtedness could make us unable to pay principal, premium, if any, and interest on the Notes and substantially decrease the market value of the Notes. If we are unable to generate sufficient cash flow and are otherwise unable to obtain funds necessary to meet required payments of principal, premium, if any, and interest on our indebtedness, or if we otherwise fail to comply with the various covenants, including financial and operating covenants, in the instruments governing our indebtedness, we could be in default under the terms of the agreements governing such indebtedness. In the event of such default, the holders of such indebtedness could elect to declare all the funds borrowed thereunder to be due and payable, together with accrued and unpaid interest, the lenders under the Credit Facility or other debt we may incur in the future could elect to terminate their commitments, cease making further loans and institute foreclosure proceedings against our assets, and we could be forced into bankruptcy or liquidation. Our ability to generate sufficient cash flow in the future is, to some extent, subject to general economic, financial, competitive, legislative and regulatory factors as well as other factors that are beyond our control. We cannot assure you that our business will generate cash flow from operations, or that future borrowings will be available to us under the Credit Facility or otherwise, in an amount sufficient to enable us to meet our payment obligations under the Notes and our other debt and to fund other liquidity needs.

        If our operating performance declines and we are not able to generate sufficient cash flow to service our debt obligations, we may in the future need to refinance or restructure our debt, including the Notes, sell assets, reduce or delay capital investments, seek to raise additional capital or seek to obtain waivers from the required lenders under the Credit Facility or the required holders of our 4.50% Notes due 2019, our 4.50% Notes due 2022 or other debt that we may incur in the future to avoid being in default. If we are unable to implement one or more of these alternatives, we may not be able to meet our payment obligations under the Notes and our other debt. If we breach our covenants under the Credit Facility, the 4.50% Notes due 2019, the 4.50% Notes due 2022 or other debt and seek a waiver, we may not be able to obtain a waiver from the required lenders or debt holders. If this occurs, we would be in default under the Credit Facility, the 4.50% Notes due 2019, the 4.50% Notes due 2022 or other debt, the lenders or debt holders could exercise their rights as described above, and we could be forced into bankruptcy or liquidation. If we are unable to repay debt, lenders having

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secured obligations could proceed against the collateral securing the debt. Because the Credit Facility has, and any future credit facilities will likely have, customary cross-default provisions, if the indebtedness under the Notes, the 4.50% Notes due 2019, the 4.50% Notes due 2022, the Credit Facility or under any future credit facility is accelerated, we may be unable to repay or finance the amounts due.

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USE OF PROCEEDS

        We estimate that the net proceeds we will receive from the sale of the $             million aggregate principal amount of the Notes in this offering will be approximately $             million, based on a public offering price of        % of par, after deducting the underwriting discount and estimated offering expenses payable by us.

        We intend to initially use the net proceeds from this offering to repay outstanding debt borrowed under our Credit Facility. However, through re-borrowing of the initial repayments under our Credit Facility, we intend to use the net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, to make investments in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, to pay our operating expenses and other cash obligations, and for general corporate purposes. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in interest bearing deposits or other short-term instruments. See "Risk Factors—Risks Relating to Our Securities—We may be unable to invest a significant portion of the net proceeds from an offering or from exiting an investment or other capital on acceptable terms, which could harm our financial condition and operating results" in the accompanying prospectus.

        On April 12, 2019, we had approximately $293.0 million outstanding under our Credit Facility. Our Credit Facility matures in September 2023, unless extended, and bears interest, at our election, on a per annum basis equal to (A)(i) the applicable LIBOR rate plus (A) 1.875% (or the applicable base rate plus 0.875%) so long as we meet certain agreed upon excess collateral and maximum leverage requirements or (B) 2.0% (or the applicable base rate plus 1.0%) if we do not meet certain agreed upon excess collateral and maximum leverage requirements. Amounts repaid under our Credit Facility will remain available for future borrowings.

        Affiliates of RBC Capital Markets, LLC, Goldman Sachs & Co. LLC, Raymond James & Associates, Inc. and BB&T Capital Markets, a division of BB&T Securities, LLC, underwriters in this offering, act as lenders and/or agents under our Credit Facility. As described above, we intend to use net proceeds of this offering to repay the outstanding indebtedness under our Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting (Conflicts of Interest)—Conflicts of Interest" below.

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CAPITALIZATION

        The following table sets forth our capitalization:

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on an as-adjusted basis giving effect to the sale of $             million aggregate principal amount of Notes in this offering, less estimated underwriting discounts and offering expenses payable by us, and the application of the proceeds thereof.

        This table should be read in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Audited Financial Statements" in this prospectus supplement.

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SELECTED FINANCIAL DATA

        The selected financial and other data as of and for the years ended December 31, 2018, 2017, 2016, 2015 and 2014 have been derived from consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. You should read this selected financial and other data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations," "Senior Securities" and the consolidated financial statements and related notes thereto in the accompanying prospectus and "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Audited Financial Statements" in this prospectus supplement.

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MANAGEMENT'S DISCUSSION AND ANALYSIS OF
FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        The following discussion should be read in conjunction with our consolidated financial statements and the notes thereto contained elsewhere in this prospectus supplement.

        Statements we make in the following discussion which express a belief, expectation or intention, as well as those that are not historical fact, are forward-looking statements that are subject to risks, uncertainties and assumptions. Our actual results, performance or achievements, or industry results, could differ materially from those we express in the following discussion as a result of a variety of factors, including the risks and uncertainties we have referred to under the headings "Supplementary Risk Factors" in this prospectus supplement and "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" in the accompanying prospectus.

ORGANIZATION

        Main Street Capital Corporation ("MSCC") is a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. The portfolio investments of MSCC and its consolidated subsidiaries are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in a variety of industry sectors. MSCC seeks to partner with entrepreneurs, business owners and management teams and generally provides "one stop" financing alternatives within its LMM portfolio. MSCC and its consolidated subsidiaries invest primarily in secured debt investments, equity investments, warrants and other securities of LMM companies based in the United States and in secured debt investments of Middle Market companies generally headquartered in the United States.

        MSCC was formed in March 2007 to operate as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSCC wholly owns several investment funds, including Main Street Mezzanine Fund, LP ("MSMF"), Main Street Capital II, LP ("MSC II") and Main Street Capital III, LP ("MSC III" and, collectively with MSMF and MSC II, the "Funds"), and each of their general partners. The Funds are each licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA"). Because MSCC is internally managed, all of the executive officers and other employees are employed by MSCC. Therefore, MSCC does not pay any external investment advisory fees, but instead directly incurs the operating costs associated with employing investment and portfolio management professionals.

        MSC Adviser I, LLC (the "External Investment Manager") was formed in November 2013 as a wholly owned subsidiary of MSCC to provide investment management and other services to parties other than MSCC and its subsidiaries or their portfolio companies ("External Parties") and receives fee income for such services. MSCC has been granted no-action relief by the Securities and Exchange Commission ("SEC") to allow the External Investment Manager to register as a registered investment adviser under the Investment Advisers Act of 1940, as amended. Since the External Investment Manager conducts all of its investment management activities for External Parties, it is accounted for as a portfolio investment of MSCC and is not included as a consolidated subsidiary of MSCC in MSCC's consolidated financial statements.

        MSCC has elected to be treated for U.S. federal income tax purposes as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"). As a result, MSCC generally will not pay corporate-level U.S. federal income taxes on any net ordinary taxable income or capital gains that it distributes to its stockholders.

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        MSCC has certain direct and indirect wholly owned subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of the Taxable Subsidiaries is to permit MSCC to hold equity investments in portfolio companies which are "pass-through" entities for tax purposes.

        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our," the "Company" and "Main Street" refer to MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries.

OVERVIEW

        Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $50 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $20 million. Our private loan ("Private Loan") portfolio investments are primarily debt securities in privately held companies which have been originated through strategic relationships with other investment funds on a collaborative basis. Private Loan investments are typically similar in size, structure, terms and conditions to investments we hold in our LMM portfolio and Middle Market portfolio.

        We seek to fill the financing gap for LMM businesses, which, historically, have had limited access to financing from commercial banks and other traditional sources. The underserved nature of the LMM creates the opportunity for us to meet the financing needs of LMM companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing options, or a "one stop" financing solution. Providing customized, "one stop" financing solutions is important to LMM portfolio companies. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Our LMM portfolio debt investments are generally secured by a first lien on the assets of the portfolio company and typically have a term of between five and seven years from the original investment date.

        Our Middle Market portfolio investments primarily consist of direct investments in or secondary purchases of interest-bearing debt securities in privately held companies that are generally larger in size than the companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have an expected duration of between three and seven years from the original investment date.

        Our Private Loan portfolio investments are primarily debt securities in privately held companies which have been originated through strategic relationships with other investment funds on a collaborative basis, and are often referred to in the debt markets as "club deals." Private Loan investments are typically similar in size, structure, terms and conditions to investments we hold in our LMM portfolio and Middle Market portfolio. Our Private Loan portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and seven years from the original investment date.

        Our other portfolio ("Other Portfolio") investments primarily consist of investments which are not consistent with the typical profiles for our LMM, Middle Market or Private Loan portfolio investments, including investments which may be managed by third parties. In our Other Portfolio, we may incur indirect fees and expenses in connection with investments managed by third parties, such as investments in other investment companies or private funds.

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        Our external asset management business is conducted through the External Investment Manager. The External Investment Manager earns management fees based on the assets of the funds under management and may earn incentive fees, or a carried interest, based on the performance of the funds managed. We have entered into an agreement with the External Investment Manager to share employees in connection with its asset management business generally, and specifically for its relationship with HMS Income Fund, Inc. ("HMS Income"). Through this agreement, we share employees with the External Investment Manager, including their related infrastructure, business relationships, management expertise and capital raising capabilities.

        The following tables provide a summary of our investments in the LMM, Middle Market and Private Loan portfolios as of December 31, 2018 and 2017 (this information excludes the Other Portfolio investments and the External Investment Manager which are discussed further below):

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        As of December 31, 2018, we had Other Portfolio investments in eleven companies, collectively totaling approximately $108.3 million in fair value and approximately $116.0 million in cost basis and which comprised approximately 4.4% of our Investment Portfolio (as defined in "Critical Accounting Policies—Basis of Presentation" below) at fair value. As of December 31, 2017, we had Other Portfolio investments in eleven companies, collectively totaling approximately $104.6 million in fair value and approximately $109.4 million in cost basis and which comprised approximately 4.8% of our Investment Portfolio at fair value.

        As previously discussed, the External Investment Manager is a wholly owned subsidiary that is treated as a portfolio investment. As of December 31, 2018, there was no cost basis in this investment and the investment had a fair value of approximately $65.7 million, which comprised approximately 2.7% of our Investment Portfolio at fair value. As of December 31, 2017, there was no cost basis in this investment and the investment had a fair value of approximately $41.8 million, which comprised approximately 1.9% of our Investment Portfolio at fair value.

        Our portfolio investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different

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regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as they are wholly owned subsidiaries of MSCC.

        The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate the identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation on our investments will also fluctuate depending upon portfolio activity, economic conditions and the performance of our individual portfolio companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

        Because we are internally managed, we do not pay any external investment advisory fees, but instead directly incur the operating costs associated with employing investment and portfolio management professionals. We believe that our internally managed structure provides us with a beneficial operating expense structure when compared to other publicly traded and privately held investment firms which are externally managed, and our internally managed structure allows us the opportunity to leverage our non-interest operating expenses as we grow our Investment Portfolio. For the years ended December 31, 2018 and 2017, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.4% and 1.6%, respectively.

        During May 2012, we entered into an investment sub-advisory agreement with HMS Adviser, LP ("HMS Adviser"), which is the investment advisor to HMS Income, a non-listed BDC, to provide certain investment advisory services to HMS Adviser. In December 2013, after obtaining required no-action relief from the SEC to allow us to own a registered investment adviser, we assigned the sub-advisory agreement to the External Investment Manager since the fees received from such arrangement could otherwise have negative consequences on our ability to meet the source-of-income requirement necessary for us to maintain our RIC tax treatment. Under the investment sub-advisory agreement, the External Investment Manager is entitled to 50% of the base management fee and the incentive fees earned by HMS Adviser under its advisory agreement with HMS Income. The External Investment Manager has conditionally agreed to waive the historical incentive fees otherwise earned. During the years ended December 31, 2018, 2017 and 2016, the External Investment Manager earned $11.6 million, $10.9 million and $9.5 million, respectively, of management fees (net of fees waived, if any) under the sub-advisory agreement with HMS Adviser.

        During April 2014, we received an exemptive order from the SEC permitting co-investments by us and HMS Income in certain negotiated transactions where co-investing would otherwise be prohibited under the 1940 Act. We have made, and in the future intend to continue to make, such co-investments with HMS Income in accordance with the conditions of the order. The order requires, among other things, that we and the External Investment Manager consider whether each such investment opportunity is appropriate for HMS Income and, if it is appropriate, to propose an allocation of the investment opportunity between us and HMS Income. Because the External Investment Manager may receive performance-based fee compensation from HMS Income, this may provide it an incentive to allocate opportunities to HMS Income instead of us. However, both we and the External Investment Manager have policies and procedures in place to manage this conflict.

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CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our consolidated financial statements are prepared in accordance with generally accepted accounting principles in the United States of America ("U.S. GAAP"). For each of the periods presented herein, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries. The Investment Portfolio, as used herein, refers to all of our investments in LMM portfolio companies, investments in Middle Market portfolio companies, Private Loan portfolio investments, Other Portfolio investments, and the investment in the External Investment Manager. Our results of operations and cash flows for the years ended December 31, 2018, 2017 and 2016 and financial position as of December 31, 2018 and 2017, are presented on a consolidated basis. The effects of all intercompany transactions between us and our consolidated subsidiaries have been eliminated in consolidation. Certain reclassifications have been made to prior period balances to conform with the current presentation.

        We are an investment company following the accounting and reporting guidance in Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC") 946, Financial Services—Investment Companies ("ASC 946"). Under ASC 946, we are precluded from consolidating other entities in which we have equity investments, including those in which we have a controlling interest, unless the other entity is another investment company. An exception to this general principle in ASC 946 occurs if we hold a controlling interest in an operating company that provides all or substantially all of its services directly to us or to any of our portfolio companies. Accordingly, as noted above, our consolidated financial statements include the financial position and operating results for the Funds and the Taxable Subsidiaries. We have determined that all of our portfolio investments do not qualify for this exception, including the investment in the External Investment Manager. Therefore, our Investment Portfolio is carried on the consolidated balance sheet at fair value with any adjustments to fair value recognized as "Net Unrealized Appreciation (Depreciation)" on the consolidated statements of operations until the investment is realized, usually upon exit, resulting in any gain or loss being recognized as a "Net Realized Gain (Loss)."

Investment Portfolio Valuation

        The most significant determination inherent in the preparation of our consolidated financial statements is the valuation of our Investment Portfolio and the related amounts of unrealized appreciation and depreciation. As of both December 31, 2018 and 2017, our Investment Portfolio valued at fair value represented approximately 96% of our total assets. We are required to report our investments at fair value. We follow the provisions of FASB ASC 820, Fair Value Measurements and Disclosures ("ASC 820"). ASC 820 defines fair value, establishes a framework for measuring fair value, establishes a fair value hierarchy based on the quality of inputs used to measure fair value and enhances disclosure requirements for fair value measurements. ASC 820 requires us to assume that the portfolio investment is to be sold in the principal market to independent market participants, which may be a hypothetical market. Market participants are defined as buyers and sellers in the principal market that are independent, knowledgeable and willing and able to transact. See "Note B.1.—Valuation of the Investment Portfolio" in the notes to consolidated financial statements for a detailed discussion of our investment portfolio valuation process and procedures.

        Due to the inherent uncertainty in the valuation process, our determination of fair value for our Investment Portfolio may differ materially from the values that would have been determined had a ready market for the securities existed. In addition, changes in the market environment, portfolio company performance and other events that may occur over the lives of the investments may cause the gains or losses ultimately realized on these investments to be materially different than the valuations

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currently assigned. We determine the fair value of each individual investment and record changes in fair value as unrealized appreciation or depreciation.

        Our Board of Directors has the final responsibility for overseeing, reviewing and approving, in good faith, our determination of the fair value for our Investment Portfolio and our valuation procedures, consistent with 1940 Act requirements. We believe our Investment Portfolio as of December 31, 2018 and 2017 approximates fair value as of those dates based on the markets in which we operate and other conditions in existence on those reporting dates.

Revenue Recognition

Interest and Dividend Income

        We record interest and dividend income on the accrual basis to the extent amounts are expected to be collected. Dividend income is recorded as dividends are declared by the portfolio company or at the point an obligation exists for the portfolio company to make a distribution. In accordance with our valuation policies, we evaluate accrued interest and dividend income periodically for collectability. When a loan or debt security becomes 90 days or more past due, and if we otherwise do not expect the debtor to be able to service all of its debt or other obligations, we will generally place the loan or debt security on non-accrual status and cease recognizing interest income on that loan or debt security until the borrower has demonstrated the ability and intent to pay contractual amounts due. If a loan or debt security's status significantly improves regarding the debtor's ability to service the debt or other obligations, or if a loan or debt security is sold or written off, we remove it from non-accrual status.

Fee Income

        We may periodically provide services, including structuring and advisory services, to our portfolio companies or other third parties. For services that are separately identifiable and evidence exists to substantiate fair value, fee income is recognized as earned, which is generally when the investment or other applicable transaction closes. Fees received in connection with debt financing transactions for services that do not meet these criteria are treated as debt origination fees and are deferred and accreted into income over the life of the financing.

Payment-in-Kind ("PIK") Interest and Cumulative Dividends

        We hold certain debt and preferred equity instruments in our Investment Portfolio that contain PIK interest and cumulative dividend provisions. The PIK interest, computed at the contractual rate specified in each debt agreement, is periodically added to the principal balance of the debt and is recorded as interest income. Thus, the actual collection of this interest may be deferred until the time of debt principal repayment. Cumulative dividends are recorded as dividend income, and any dividends in arrears are added to the balance of the preferred equity investment. The actual collection of these dividends in arrears may be deferred until such time as the preferred equity is redeemed or sold. To maintain RIC tax treatment (as discussed below), these non-cash sources of income may need to be paid out to stockholders in the form of distributions, even though we may not have collected the PIK interest and cumulative dividends in cash. We stop accruing PIK interest and cumulative dividends and write off any accrued and uncollected interest and dividends in arrears when we determine that such PIK interest and dividends in arrears are no longer collectible. For the years ended December 31, 2018, 2017 and 2016, (i) approximately 1.0%, 2.4%, and 3.6%, respectively, of our total investment income was attributable to PIK interest income not paid currently in cash and (ii) approximately 1.0%, 1.6%, and 1.2%, respectively, of our total investment income was attributable to cumulative dividend income not paid currently in cash.

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Share-Based Compensation

        We account for our share-based compensation plans using the fair value method, as prescribed by ASC 718, Compensation—Stock Compensation. Accordingly, for restricted stock awards, we measure the grant date fair value based upon the market price of our common stock on the date of the grant and amortize the fair value of the awards as share-based compensation expense over the requisite service period, which is generally the vesting term.

        We have also adopted Accounting Standards Update ("ASU") 2016-09, Compensation—Stock Compensation: Improvements to Employee Share-Based Payment Accounting, which requires that all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) be recognized as income tax expense or benefit in the income statement and not delay recognition of a tax benefit until the tax benefit is realized through a reduction to taxes payable. The tax effects of exercised or vested awards should be treated as discrete items in the reporting period in which they occur. Additionally, we have elected to account for forfeitures as they occur.

Income Taxes

        MSCC has elected to be treated for U.S. federal income tax purposes as a RIC. MSCC's taxable income includes the taxable income generated by MSCC and certain of its subsidiaries, including the Funds, which are treated as disregarded entities for tax purposes. As a RIC, MSCC generally will not pay corporate-level U.S. federal income taxes on any net ordinary taxable income or capital gains that MSCC distributes to its stockholders. MSCC must generally distribute at least 90% of its "investment company taxable income" (which is generally its net ordinary taxable income and realized net short-term capital gains in excess of realized net long-term capital losses) and 90% of its tax-exempt income to maintain its RIC status (pass-through tax treatment for amounts distributed). As part of maintaining RIC status, undistributed taxable income (subject to a 4% non-deductible U.S. federal excise tax) pertaining to a given fiscal year may be distributed up to 12 months subsequent to the end of that fiscal year, provided such dividends are declared on or prior to the later of (i) filing of the U.S. federal income tax return for the applicable fiscal year or (ii) the fifteenth day of the ninth month following the close of the year in which such taxable income was generated.

        The Taxable Subsidiaries primarily hold certain portfolio investments for us. The Taxable Subsidiaries permit us to hold equity investments in portfolio companies which are "pass-through" entities for tax purposes and to continue to comply with the "source-of-income" requirements contained in the RIC tax provisions of the Code. The Taxable Subsidiaries are consolidated with us for U.S. GAAP financial reporting purposes, and the portfolio investments held by the Taxable Subsidiaries are included in our consolidated financial statements as portfolio investments and recorded at fair value. The Taxable Subsidiaries are not consolidated with MSCC for income tax purposes and may generate income tax expense, or benefit, and tax assets and liabilities, as a result of their ownership of certain portfolio investments. The taxable income, or loss, of the Taxable Subsidiaries may differ from their book income, or loss, due to temporary book and tax timing differences and permanent differences. The Taxable Subsidiaries are each taxed at their normal corporate tax rates based on their taxable income. The income tax expense, or benefit, if any, and the related tax assets and liabilities, of the Taxable Subsidiaries are reflected in our consolidated financial statements.

        The External Investment Manager is an indirect wholly owned subsidiary of MSCC owned through a Taxable Subsidiary and is a disregarded entity for tax purposes. The External Investment Manager has entered into a tax sharing agreement with its Taxable Subsidiary owner. Since the External Investment Manager is accounted for as a portfolio investment of MSCC and is not included as a consolidated subsidiary of MSCC in MSCC's consolidated financial statements, and as a result of the tax sharing agreement with its Taxable Subsidiary owner, for its stand-alone financial reporting purposes the External Investment Manager is treated as if it is taxed at normal corporate tax rates based on its

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taxable income and, as a result of its activities, may generate income tax expense or benefit. The income tax expense, or benefit, if any, and the related tax assets and liabilities, of the External Investment Manager are reflected in the External Investment Manager's separate financial statements.

        In December 2017, the "Tax Cuts and Jobs Act" legislation was enacted. The Tax Cuts and Jobs Act includes significant changes to the U.S. corporate tax system, including a U.S. federal corporate income tax rate reduction from 35% to 21% and other changes. ASC 740, Income Taxes, requires the effects of changes in tax rates and laws on deferred tax balances to be recognized in the period in which the legislation was enacted. As such, we have accounted for the tax effects as a result of the enactment of the Tax Cuts and Jobs Act beginning with the period ended December 31, 2017.

        The Taxable Subsidiaries and the External Investment Manager use the liability method in accounting for income taxes. Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and their reported amounts in the consolidated financial statements, using statutory tax rates in effect for the year in which the temporary differences are expected to reverse. A valuation allowance is provided, if necessary, against deferred tax assets when it is more likely than not that some portion or all of the deferred tax asset will not be realized.

        Taxable income generally differs from net income for financial reporting purposes due to temporary and permanent differences in the recognition of income and expenses. Taxable income generally excludes net unrealized appreciation or depreciation, as investment gains or losses are not included in taxable income until they are realized.

INVESTMENT PORTFOLIO COMPOSITION

        Our LMM portfolio investments primarily consist of secured debt, equity warrants and direct equity investments in privately held, LMM companies based in the United States. Our LMM portfolio companies generally have annual revenues between $10 million and $150 million, and our LMM investments generally range in size from $5 million to $50 million. The LMM debt investments are typically secured by either a first or second priority lien on the assets of the portfolio company, can include either fixed or floating rate terms and generally have a term of between five and seven years from the original investment date. In most LMM portfolio investments, we receive nominally priced equity warrants and/or make direct equity investments in connection with a debt investment.

        Our Middle Market portfolio investments primarily consist of direct investments in or secondary purchases of interest-bearing debt securities in privately held companies based in the United States that are generally larger in size than the companies included in our LMM portfolio. Our Middle Market portfolio companies generally have annual revenues between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $20 million. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and seven years from the original investment date.

        Our Private Loan portfolio investments are primarily debt securities in privately held companies which have been originated through strategic relationships with other investment funds on a collaborative basis, and are often referred to in the debt markets as "club deals." Private Loan investments are typically similar in size, structure, terms and conditions to investments we hold in our LMM portfolio and Middle Market portfolio. Our Private Loan portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and seven years from the original investment date.

        Our Other Portfolio investments primarily consist of investments which are not consistent with the typical profiles for LMM, Middle Market and Private Loan portfolio investments, including investments which may be managed by third parties. In the Other Portfolio, we may incur indirect fees and expenses in connection with investments managed by third parties, such as investments in other investment companies or private funds.

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        Our external asset management business is conducted through the External Investment Manager. The External Investment Manager earns management fees based on the assets of the funds under management and may earn incentive fees, or a carried interest, based on the performance of the funds managed. We have entered into an agreement with the External Investment Manager to share employees in connection with its asset management business generally, and specifically for its relationship with HMS Income. Through this agreement, we share employees with the External Investment Manager, including their related infrastructure, business relationships, management expertise and capital raising capabilities, and we allocate the related expenses to the External Investment Manager pursuant to the sharing agreement. Our total expenses for the years ended December 31, 2018, 2017 and 2016 are net of expenses allocated to the External Investment Manager of $6.8 million, $6.4 million and $5.1 million, respectively. The External Investment Manager earns management fees based on the assets of the funds under management and may earn incentive fees, or a carried interest, based on the performance of the funds managed. The total contribution of the External Investment Manager to our net investment income consists of the combination of the expenses allocated to the External Investment Manager and the dividend income received from the External Investment Manager. For the years ended December 31, 2018, 2017 and 2016, the total contribution to our net investment income was $10.6 million, $9.4 million and $7.9 million, respectively.

        The following tables summarize the composition of our total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments at cost and fair value by type of investment as a percentage of the total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments as of December 31, 2018 and 2017 (this information excludes the Other Portfolio investments and the External Investment Manager).

 

        Our LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments carry a number of risks including: (1) investing in companies which may have limited operating histories and financial resources; (2) holding investments that generally are not publicly traded and which may be subject to legal and other restrictions on resale; and (3) other risks common to investing in below investment grade debt and equity investments in our Investment Portfolio. Please see "Risk Factors—Risks Related to Our Investments" in the accompanying prospectus and "Supplementary Risk Factors" in this prospectus supplement for a more complete discussion of the risks involved with investing in our Investment Portfolio.

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PORTFOLIO ASSET QUALITY

        We utilize an internally developed investment rating system to rate the performance of each LMM portfolio company and to monitor our expected level of returns on each of our LMM investments in relation to our expectations for the portfolio company. The investment rating system takes into consideration various factors, including each investment's expected level of returns, the collectability of our debt investments and the ability to receive a return of the invested capital in our equity investments, comparisons to competitors and other industry participants, the portfolio company's future outlook and other factors that are deemed to be significant to the portfolio company.

        As of December 31, 2018, our total Investment Portfolio had six investments on non-accrual status, which comprised approximately 1.3% of its fair value and 3.9% of its cost. As of December 31, 2017, our total Investment Portfolio had five investments on non-accrual status, which comprised approximately 0.2% of its fair value and 2.3% of its cost.

        The operating results of our portfolio companies are impacted by changes in the broader fundamentals of the United States economy. In the event that the United States economy contracts, it is likely that the financial results of small to mid-sized companies, like those in which we invest, could experience deterioration or limited growth from current levels, which could ultimately lead to difficulty in meeting their debt service requirements, to an increase in defaults on our debt investments or in realized losses on our investments and to difficulty in maintaining historical dividend payment rates and unrealized appreciation on our equity investments. Consequently, we can provide no assurance that the performance of certain portfolio companies will not be negatively impacted by economic cycles or other conditions, which could also have a negative impact on our future results.

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DISCUSSION AND ANALYSIS OF RESULTS OF OPERATIONS

Comparison of the years ended December 31, 2018 and 2017

 

Investment Income

        For the year ended December 31, 2018, total investment income was $233.4 million, a 13% increase over the $205.7 million of total investment income for the corresponding period of 2017. This comparable period increase was principally attributable to (i) a $15.2 million net increase in interest

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income primarily related to higher average levels of Investment Portfolio debt investments and an increase in their average effective yields, partially offset by decreases in interest income associated with activity from portfolio companies that is considered to be less consistent on a recurring basis or non-recurring and prepayment, repricing and other activities involving existing Investment Portfolio debt investments, (ii) a $11.8 million increase in dividend income from Investment Portfolio equity investments and (iii) a $0.7 million increase in fee income. The $27.6 million increase in total investment income in the year ended December 31, 2018 includes $6.3 million related to elevated dividend income activity from certain Investment Portfolio equity investments that is considered to be less consistent on a recurring basis or non-recurring, partially offset by (i) a decrease of $2.7 million related to interest income activity from portfolio companies that is considered to be less consistent on a recurring basis or non-recurring and (ii) a decrease of $2.5 million related to lower accelerated prepayment, repricing and other activity for certain Investment Portfolio debt investments, in each case when compared to the same period in 2017.

Expenses

        For the year ended December 31, 2018, total expenses increased to $76.7 million from $70.4 million for the corresponding period of 2017. This comparable period increase in operating expenses was principally attributable to (i) a $7.0 million increase in interest expense, primarily due to an $8.0 million increase as a result of the issuance of our 4.50% Notes due 2022 in November 2017, with the remainder of the difference from prior year due to the higher average balance of SBIC debentures outstanding and an increase in both the average balance outstanding and the interest rate on our multi-year revolving credit facility (the "Credit Facility"), with these increases partially offset by a decrease from the redemption of the 6.125% Notes effective April 1, 2018, and (ii) a $0.4 million increase in compensation expense related to increases in the number of personnel, base compensation levels and incentive compensation accruals, with these increases partially offset by (i) a $0.9 million decrease in share-based compensation expense, (ii) a decrease of $0.9 million related to an additional decrease in incentive compensation accruals and (iii) a $0.4 million increase in the expenses allocated to the External Investment Manager as a result of elevated non-recurring strategic activities at the External Investment Manager during the year ended December 31, 2018. The $0.4 million increase in compensation expense is after (i) a $1.5 million decrease that is considered to be a one-time non-recurring benefit due to the conversion of a cash bonus to an expected non-cash restricted stock grant for an executive that will be amortized as non-cash, share-based compensation expense over the future service period and (ii) a $0.4 million decrease as a result of the decrease in the fair value of our deferred compensation plan assets. The ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets for the year ended December 31, 2018 was 1.4% on an annualized basis compared to 1.6% for the year ended December 31, 2017.

Net Investment Income

        Net investment income for the year ended December 31, 2018 was $156.6 million, or a 16% increase, compared to net investment income of $135.4 million for the corresponding period of 2017. The increase in net investment income was principally attributable to the increase in total investment income, partially offset by higher operating expenses both as discussed above.

Distributable Net Investment Income

        For the year ended December 31, 2018, distributable net investment income increased 14% to $165.8 million, or $2.76 per share, compared with $145.4 million, or $2.56 per share, in the corresponding period of 2017. The increase in distributable net investment income was primarily due to the higher level of total investment income, partially offset by higher operating expenses both as discussed above. Distributable net investment income on a per share basis for the year ended

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December 31, 2018 reflects (i) a consistent level of income per share from the comparable period in 2017 attributable to the net effect of the elevated dividend income activity, offset by the decreases in interest income associated with the comparable levels of activity from portfolio companies that is considered to be less consistent on a recurring basis or non-recurring and accelerated prepayment, repricing and other income activity considered non-recurring, as discussed above, (ii) an increase of $0.03 per share due to the non-recurring benefit to compensation expense and the decrease in the fair value of the deferred compensation plan assets, both as discussed above, and (iii) a greater number of average shares outstanding compared to the corresponding period in 2017 primarily due to shares issued through the ATM Program (as defined in "—Liquidity and Capital Resources—Capital Resources" below), shares issued pursuant to our equity incentive plans and shares issued pursuant to our dividend reinvestment plan.

Net Increase in Net Assets Resulting from Operations

        The net increase in net assets resulting from operations during the year ended December 31, 2018 was $168.2 million, or $2.80 per share, compared with $170.6 million, or $3.01 per share, during the year ended December 31, 2017. This $2.4 million decrease from the prior year was primarily the result of (i) a $29.5 million decrease in net unrealized appreciation from portfolio investments and SBIC debentures, including the impact of accounting reversals relating to realized gains/income (losses), and (ii) a $14.8 million decrease in the net realized gain from investments, with these decreases partially offset by (i) a $21.3 million increase in net investment income as discussed above, (ii) a $18.3 million decrease in the income tax provision and (iii) a $2.3 million improvement in the net realized loss on extinguishment of debt. The net realized gain from investments of $1.3 million for the year ended December 31, 2018 was primarily the result of (i) the net realized gain of $13.7 million resulting from the net effect of gains on the exits of six LMM investments, partially offset by losses on the exits of four LMM investments and other activity in the LMM portfolio, (ii) the realized gains of $6.1 million due to activity in our Other Portfolio and (iii) the realized gains of $2.5 million in our Private Loan portfolio, with the effect of these net realized gains partially offset by the net realized loss of $20.9 million in our Middle Market portfolio, which is primarily the result of (i) the realized losses of $17.6 million on the restructures of two Middle Market investments and (ii) the realized losses of $4.4 million on the exits of two Middle Market investments.

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        The following table provides a summary of the total net unrealized appreciation of $19.3 million for the year ended December 31, 2018:

        The income tax provision for the year ended December 31, 2018 of $6.2 million principally consisted of a deferred tax provision of $5.8 million, which is primarily the result of the net activity relating to our portfolio investments held in our Taxable Subsidiaries, including changes in loss carryforwards, changes in net unrealized appreciation/depreciation and other temporary book-tax differences, and other current tax expense of $0.4 million.

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Comparison of the years ended December 31, 2017 and 2016

 

Investment Income

        For the year ended December 31, 2017, total investment income was $205.7 million, a 15% increase over the $178.3 million of total investment income for the corresponding period of 2016. This comparable period increase was principally attributable to (i) a $23.2 million increase in interest income primarily related to higher average levels of portfolio debt investments and increased activities involving

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existing Investment Portfolio debt investments, (ii) a $2.5 million increase in dividend income from Investment Portfolio equity investments and (iii) a $1.8 million increase in fee income. The $27.4 million increase in total investment income in the year ended December 31, 2017 includes (i) an increase of $6.7 million related to higher accelerated prepayment, repricing and other activity for certain portfolio debt investments when compared to the same period in 2016, (ii) an increase of $2.7 million related to interest income activity from portfolio companies that is considered to be less consistent on a recurring basis or non-recurring during the period when compared to the same period in 2016 and (iii) includes $1.7 million related to dividend income activity from portfolio companies that is considered to be less consistent on a recurring basis or non-recurring which is consistent with the amount from such dividend income activity in the same period in 2016.

Expenses

        For the year ended December 31, 2017, total expenses increased to $70.4 million from $62.5 million for the corresponding period of 2016. This comparable period increase in operating expenses was principally attributable to (i) a $2.8 million increase in interest expense, primarily due to (a) a $1.4 million increase on the Credit Facility due to the higher average interest rate during 2017, (b) a $0.9 million increase due to the issuance of our 4.50% Notes due 2022 in November 2017 and (c) a $0.5 million increase due to the higher average balance of SBIC debentures outstanding, (ii) a $2.4 million increase in general and administrative expenses, including approximately $0.6 million related to non-recurring professional fees and other expenses incurred on certain potential new portfolio investment opportunities which were terminated during the due diligence and legal documentation processes, (iii) a $2.2 million increase in compensation expense related to increases in the number of personnel, base compensation levels and incentive compensation accruals and (iv) a $1.7 million increase in share-based compensation expense, with these increases partially offset by a $1.3 million increase in the expenses allocated to the External Investment Manager, in each case when compared to the same period in the prior year. For the years ended December 31, 2017 and 2016, the ratio of our total operating expenses, excluding interest expense and the non-recurring professional fees and other expenses discussed above as a percentage of our quarterly average total assets was 1.5%. Including the effect of the non-recurring expenses, the ratio for the year ended December 31, 2017 was 1.6%.

Net Investment Income

        Net investment income for the year ended December 31, 2017 was $135.4 million, or a 17% increase, compared to net investment income of $115.8 million for the corresponding period of 2016. The increase in net investment income was principally attributable to the increase in total investment income, partially offset by higher operating expenses both as discussed above.

Distributable Net Investment Income

        For the year ended December 31, 2017, distributable net investment income increased 17% to $145.4 million, or $2.56 per share, compared with $124.1 million, or $2.39 per share in 2016. The increase in distributable net investment income was primarily due to the higher level of total investment income, partially offset by higher operating expenses both as discussed above. Distributable net investment income on a per share basis for the year ended December 31, 2017 reflects an (i) increase of approximately $0.16 per share from the comparable period in 2016 attributable to the net increase in the comparable levels of accelerated prepayment, repricing and other, unusual activity for certain Investment Portfolio debt investments and (ii) a greater number of average shares outstanding compared to the corresponding period in 2016 primarily due to shares issued through the ATM Program (as defined in "—Liquidity and Capital Resources—Capital Resources" below), shares

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issued pursuant to our equity incentive plans and shares issued pursuant to our dividend reinvestment plan.

Net Increase in Net Assets Resulting from Operations

        The net increase in net assets resulting from operations during the year ended December 31, 2017 was $170.6 million, or $3.01 per share, compared with $138.9 million, or $2.67 per share, during the year ended December 31, 2016. This $31.7 million increase from the prior year was primarily the result of (i) a $56.3 million improvement in net unrealized appreciation (depreciation) from portfolio investments and SBIC debentures, including the impact of accounting reversals relating to realized gains/income (losses), from net unrealized depreciation of $7.5 million for the year ended December 31, 2016 to net unrealized appreciation of $48.8 million for the year ended December 31, 2017, which includes the impact of approximately $15.0 million of unrealized appreciation in the LMM equity portfolio related to the enactment of the Tax Cuts and Jobs Act (see further discussion above in "—Critical Accounting Policies—Income Taxes") and (ii) a $19.6 million increase in net investment income as discussed above, with these increases partially offset by (i) a $25.7 million change in the income tax benefit (provision) from an income tax benefit of $1.2 million for the year ended December 31, 2016 to an income tax provision of $24.5 million for the year ended December 31, 2017, (ii) a $13.2 million decrease in the net realized gain from investments to a total net realized gain from investments of $16.2 million for the year ended December 31, 2017 and (iii) a $5.2 million realized loss on the repayment of SBIC debentures outstanding at MSC II which had previously been accounted for on the fair value method of accounting. The net realized gain from investments of $16.2 million for the year ended December 31, 2017 was primarily the result of (i) the net realized gain of $11.8 million resulting from gains on the exits of five LMM investments and losses on the exits of four LMM investments, (ii) realized gains of $9.3 million due to activity in our Other Portfolio, (iii) net realized gains of $3.0 million in our Private Loan portfolio resulting from gains on the exits of two Private Loan investments and a loss on the restructure of a Private Loan investment, (iv) realized gains of $2.1 million related to other activity in the LMM portfolio and (v) the net realized loss of $9.8 million in our Middle Market portfolio, which is primarily the result of (a) realized losses of $7.9 million on the exits of two Middle Market investments and (b) the realized loss of $3.5 million on the restructure of a Middle Market investment, with these changes partially offset by $1.5 million of net realized gains on other activity in our Middle Market portfolio. The realized loss of $5.2 million on the repayment of SBIC debentures is related to the previously recognized bargain purchase gain resulting from recording the MSC II debentures at fair value on the date of the acquisition of the majority of the equity interests of MSC II in 2010. The effect of the realized loss is offset by the reversal of all previously recognized unrealized depreciation on these SBIC debentures due to fair value adjustments since the date of the acquisition.

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        The following table provides a summary of the total net unrealized appreciation of $48.8 million for the year ended December 31, 2017:

        The income tax provision for the year ended December 31, 2017 of $24.5 million principally consisted of a deferred tax provision of $19.3 million, which is primarily the result of the net activity relating to our portfolio investments held in our Taxable Subsidiaries, including changes in loss carryforwards, changes in net unrealized appreciation/depreciation and other temporary book-tax differences, and other current tax expense of $5.2 million related to (i) a $1.9 million accrual for excise tax on our estimated undistributed taxable income and (ii) current tax expense of $3.3 million related to accruals for U.S. federal and state income taxes.

Liquidity and Capital Resources

Cash Flows

        For the year ended December 31, 2018, we experienced a net increase in cash and cash equivalents in the amount of approximately $2.7 million, which is the net result of approximately $109.1 million of cash used in our operating activities and approximately $111.7 million of cash provided by our financing activities.

        During the year ended December 31, 2018, $109.1 million of cash was used in our operating activities, which resulted primarily from (i) cash flows we generated from the operating profits earned through our operating activities totaling $149.8 million, which is our $165.8 million of distributable net investment income, excluding the non-cash effects of the accretion of unearned income of $14.7 million, payment-in-kind interest income of $2.3 million, cumulative dividends of $2.3 million and the

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amortization expense for deferred financing costs of $3.3 million, and (ii) cash uses totaling $963.4 million, which principally consisted of $962.5 million for the funding of new portfolio company investments and settlement of accruals for portfolio investments existing as of December 31, 2017 and $0.9 million related to decreases in payables and accruals and (iii) cash proceeds totaling $704.6 million from $703.2 million in cash proceeds from the sales and repayments of debt investments and sales of and return on capital of equity investments and $1.4 million related to decreases in other assets.

        During the year ended December 31, 2018, $111.7 million in cash was provided by financing activities, which principally consisted of (i) $237.0 million in net cash proceeds from the Credit Facility (ii) $78.4 million in net cash proceeds from the ATM Program (described below), and (iii) $54.0 million in cash proceeds from issuance of SBIC debentures, partially offset by (i) $156.0 million in cash dividends paid to stockholders, (ii) $90.7 million in redemption of 6.125% Notes, (iii) $4.0 million in repayment of SBIC debentures, (iv) $4.1 million for purchases of vested restricted stock from employees to satisfy their tax withholding requirements upon the vesting of such restricted stock and (v) $2.9 million for payment of deferred debt issuance costs, SBIC debenture fees and other costs.

        For the year ended December 31, 2017, we experienced a net increase in cash and cash equivalents in the amount of approximately $27.0 million, which is the result of approximately $72.9 million of cash provided by our operating activities and approximately $45.9 million of cash used in financing activities.

        During the year ended December 31, 2017, $72.9 million of cash was provided by our operating activities, which resulted primarily from (i) cash flows we generated from the operating profits earned through our operating activities totaling $123.1 million, which is our $145.4 million of distributable net investment income, excluding the non-cash effects of the accretion of unearned income of $17.0 million, payment-in-kind interest income of $4.9 million, cumulative dividends of $3.2 million and the amortization expense for deferred financing costs of $2.8 million, (ii) cash uses totaling $876.7 million for the funding of new portfolio company investments and settlement of accruals for portfolio investments existing as of December 31, 2016, and (iii) cash proceeds totaling $826.5 million from (a) $819.4 million in cash proceeds from the sales and repayments of debt investments and sales of and return on capital of equity investments, (b) $4.5 million related to decreases in other assets and (c) $2.6 million related to increases in payables and accruals.

        During the year ended December 31, 2017, $45.9 million in cash was used in financing activities, which principally consisted of (i) $150.9 million in net cash proceeds from the ATM Program (described below), (ii) $185.0 million in cash proceeds from the issuance of 4.50% Notes due 2022 in November 2017 and (iii) $81.0 million in cash proceeds from issuance of SBIC debentures, partially offset by (i) $279.0 million in net repayments on the Credit Facility and (ii) $148.4 million in cash dividends paid to stockholders, (iii) $25.2 million in repayment of SBIC debentures, (iii) $4.4 million for purchases of vested restricted stock from employees to satisfy their tax withholding requirements upon the vesting of such restricted stock and (iv) $5.9 million for payment of deferred debt issuance costs, SBIC debenture fees and other costs.

Capital Resources

        As of December 31, 2018, we had $54.2 million in cash and cash equivalents and $404.0 million of unused capacity under the Credit Facility, which we maintain to support our investment and operating activities. As of December 31, 2018, our net asset value totaled $1,476.0 million, or $24.09 per share.

        The Credit Facility, which provides additional liquidity to support our investment and operational activities, was amended and restated during 2018 to provide for an increase in total commitments from $585.0 million to $705.0 million and to increase the diversified group of lenders to eighteen, eliminate interest rate adjustments previously subject to our maintenance of an investment grade rating and extend the final maturity by two years to September 2023. The amended Credit Facility also contains an upsized accordion feature which allows us to increase the total commitments under the facility to up to $800.0 million from new and existing lenders on the same terms and conditions as the existing commitments.

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        Borrowings under the Credit Facility bear interest, subject to our election, on a per annum basis at a rate equal to the applicable LIBOR rate (2.5% as of December 31, 2018) plus (i) 1.875% (or the applicable base rate (Prime Rate of 5.5% as of December 31, 2018) plus 0.875%) as long as we meet certain agreed upon excess collateral and maximum leverage requirements or (ii) 2.0% (or the applicable base rate plus 1.0%) otherwise. We pay unused commitment fees of 0.25% per annum on the unused lender commitments under the Credit Facility. The Credit Facility is secured by a first lien on the assets of MSCC and its subsidiaries, excluding the equity ownership or assets of the Funds and the External Investment Manager. The Credit Facility contains certain affirmative and negative covenants, including but not limited to: (i) maintaining a minimum availability of at least 10% of the borrowing base, (ii) maintaining an interest coverage ratio of at least 2.0 to 1.0, (iii) maintaining an asset coverage ratio (tangible net worth to Credit Facility borrowings) of at least 1.5 to 1.0 and (iv) maintaining a minimum tangible net worth. The Credit Facility is provided on a revolving basis through its final maturity date in September 2023, and contains two, one-year extension options which could extend the final maturity by up to two years, subject to certain conditions, including lender approval. As of December 31, 2018, we had $301.0 million in borrowings outstanding under the Credit Facility, the interest rate on the Credit Facility was 4.2% and we were in compliance with all financial covenants of the Credit Facility.

        Through the Funds, we have the ability to issue SBIC debentures guaranteed by the SBA at favorable interest rates and favorable terms and conditions. Under existing SBA regulations, SBA approved SBICs under common control have the ability to issue debentures guaranteed by the SBA up to a regulatory maximum amount of $350.0 million. Through the Funds, we have an effective maximum amount of $346.0 million following the prepayment of $4.0 million of existing SBIC debentures as discussed below. During the year ended December 31, 2018, we issued $54.0 million of SBIC debentures and opportunistically prepaid $4.0 million of our existing SBIC debentures as part of an effort to manage the maturity dates of our oldest SBIC debentures. Debentures guaranteed by the SBA have fixed interest rates that equal prevailing 10-year Treasury Note rates plus a market spread and have a maturity of ten years with interest payable semiannually. The principal amount of the debentures is not required to be paid before maturity, but may be pre-paid at any time with no prepayment penalty. We expect to issue new SBIC debentures under the SBIC program in the future in an amount up to the regulatory maximum amount for affiliated SBIC funds. As of December 31, 2018, through our three wholly owned SBICs, we had $345.8 million of outstanding SBIC debentures guaranteed by the SBA, which bear a weighted-average annual fixed interest rate of approximately 3.7%, paid semiannually, and mature ten years from issuance. The first maturity related to our SBIC debentures occurs in 2019, and the weighted-average remaining duration is approximately 5.6 years as of December 31, 2018.

        In April 2013, we issued $92.0 million, including the underwriters' full exercise of their over-allotment option, in aggregate principal amount of the 6.125% Notes (the "6.125% Notes"). The 6.125% Notes bore interest at a rate of 6.125% per year payable quarterly on January 1, April 1, July 1 and October 1 of each year. The total net proceeds to us from the 6.125% Notes, after underwriting discounts and estimated offering expenses payable, were approximately $89.0 million. On April 2, 2018, we redeemed the entire principal amount of the issued and outstanding 6.125% Notes effective April 1, 2018 (the "Redemption Date"). The 6.125% Notes were redeemed at par value, plus the accrued and unpaid interest thereon from January 1, 2018, through, but excluding, the Redemption Date. As part of the redemption, we recognized a realized loss on extinguishment of debt of $1.5 million in the second quarter of 2018 related to the write-off of the related unamortized deferred financing costs.

        In November 2014, we issued $175.0 million in aggregate principal amount of 4.50% unsecured notes due 2019 (the "4.50% Notes due 2019") at an issue price of 99.53%. The 4.50% Notes due 2019 are unsecured obligations and rank pari passu with our current and future unsecured indebtedness; senior to any of our future indebtedness that expressly provides it is subordinated to the 4.50% Notes

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due 2019; effectively subordinated to all of our existing and future secured indebtedness, to the extent of the value of the assets securing such indebtedness, including borrowings under our Credit Facility; and structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, including without limitation, the indebtedness of the Funds. The 4.50% Notes due 2019 mature on December 1, 2019, and may be redeemed in whole or in part at any time at our option subject to certain make-whole provisions. The 4.50% Notes due 2019 bear interest at a rate of 4.50% per year payable semiannually on June 1 and December 1 of each year. We may from time to time repurchase 4.50% Notes due 2019 in accordance with the 1940 Act and the rules promulgated thereunder. As of December 31, 2018, the outstanding balance of the 4.50% Notes due 2019 was $175.0 million.

        The indenture governing the 4.50% Notes due 2019 (the "4.50% Notes due 2019 Indenture") contains certain covenants, including covenants requiring our compliance with (regardless of whether we are subject to) the asset coverage requirements set forth in Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act, as well as covenants requiring us to provide financial information to the holders of the 4.50% Notes due 2019 and the Trustee if we cease to be subject to the reporting requirements of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). These covenants are subject to limitations and exceptions that are described in the 4.50% Notes due 2019 Indenture.

        In November 2017, we issued $185.0 million in aggregate principal amount of 4.50% unsecured notes due 2022 (the "4.50% Notes due 2022") at an issue price of 99.16%. The 4.50% Notes due 2022 are unsecured obligations and rank pari passu with our current and future unsecured indebtedness; senior to any of our future indebtedness that expressly provides it is subordinated to the 4.50% Notes due 2022; effectively subordinated to all of our existing and future secured indebtedness, to the extent of the value of the assets securing such indebtedness, including borrowings under our Credit Facility; and structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, including without limitation, the indebtedness of the Funds. The 4.50% Notes due 2022 mature on December 1, 2022, and may be redeemed in whole or in part at any time at our option subject to certain make-whole provisions. The 4.50% Notes due 2022 bear interest at a rate of 4.50% per year payable semiannually on June 1 and December 1 of each year. We may from time to time repurchase 4.50% Notes due 2022 in accordance with the 1940 Act and the rules promulgated thereunder. As of December 31, 2018, the outstanding balance of the 4.50% Notes due 2022 was $185.0 million.

        The indenture governing the 4.50% Notes due 2022 (the "4.50% Notes due 2022 Indenture") contains certain covenants, including covenants requiring our compliance with (regardless of whether we are subject to) the asset coverage requirements set forth in Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act, as well as covenants requiring us to provide financial information to the holders of the 4.50% Notes due 2022 and the Trustee if we cease to be subject to the reporting requirements of the Exchange Act. These covenants are subject to limitations and exceptions that are described in the 4.50% Notes due 2022 Indenture.

        We maintain a program with certain selling agents through which we can sell shares of our common stock by means of at-the-market offerings from time to time (the "ATM Program").

        During the year ended December 31, 2016, we sold 3,324,646 shares of our common stock at a weighted-average price of $34.17 per share and raised $113.6 million of gross proceeds under the ATM Program. Net proceeds were $112.0 million after commissions to the selling agents on shares sold and offering costs.

        During the year ended December 31, 2017, we sold 3,944,972 shares of our common stock at a weighted-average price of $38.72 per share and raised $152.8 million of gross proceeds under the ATM

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Program. Net proceeds were $150.9 million after commissions to the selling agents on shares sold and offering costs.

        During the year ended December 31, 2018, we sold 2,060,019 shares of our common stock at a weighted-average price of $38.48 per share and raised $79.3 million of gross proceeds under the ATM Program. Net proceeds were $78.0 million after commissions to the selling agents on shares sold and offering costs. As of December 31, 2018, 2,994,469 shares remained available for sale under the ATM Program.

        We anticipate that we will continue to fund our investment activities through existing cash and cash equivalents, cash flows generated through our ongoing operating activities, utilization of available borrowings under our Credit Facility, and a combination of future issuances of debt and equity capital. Our primary uses of funds will be investments in portfolio companies, operating expenses and cash distributions to holders of our common stock.

        We periodically invest excess cash balances into marketable securities and idle funds investments. The primary investment objective of marketable securities and idle funds investments is to generate incremental cash returns on excess cash balances prior to utilizing those funds for investment in our LMM, Middle Market and Private Loan portfolio investments. Marketable securities and idle funds investments generally consist of debt investments, independently rated debt investments, certificates of deposit with financial institutions, diversified bond funds and publicly traded debt and equity investments.

        If our common stock trades below our net asset value per share, we will generally not be able to issue additional common stock at the market price unless our stockholders approve such a sale and our Board of Directors makes certain determinations. We did not seek stockholder authorization to sell shares of our common stock below the then current net asset value per share of our common stock at our 2018 annual meeting of stockholders because our common stock price per share had been trading significantly above the net asset value per share of our common stock since 2011. We would therefore need future approval from our stockholders to issue shares below the then current net asset value per share.

        In order to satisfy the Code requirements applicable to a RIC, we intend to distribute to our stockholders, after consideration and application of our ability under the Code to carry forward certain excess undistributed taxable income from one tax year into the next tax year, substantially all of our taxable income. In addition, as a BDC, we generally are required to meet a coverage ratio of total assets to total senior securities, which include borrowings and any preferred stock we may issue in the future, of at least 200% (or 150% if certain requirements are met). This requirement limits the amount that we may borrow. In January 2008, we received an exemptive order from the SEC to exclude SBA-guaranteed debt securities issued by MSMF and any other wholly owned subsidiaries of ours which operate as SBICs from the asset coverage requirements of the 1940 Act as applicable to us, which, in turn, enables us to fund more investments with debt capital.

        Although we have been able to secure access to additional liquidity, including through the Credit Facility, public debt issuances, leverage available through the SBIC program and equity offerings, there is no assurance that debt or equity capital will be available to us in the future on favorable terms, or at all.

Recently Issued or Adopted Accounting Standards

        In May 2014, the FASB issued Accounting Standards Update ("ASU") 2014-09, Revenue from Contracts with Customers (Topic 606). ASU 2014-09 supersedes the revenue recognition requirements under ASC 605, Revenue Recognition, and most industry-specific guidance throughout the Industry Topics of the ASC. The core principle of the guidance is that an entity should recognize revenue to

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depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which an entity expects to be entitled in exchange for those goods or services. Under the guidance, an entity is required to perform the following five steps: (1) identify the contract(s) with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when (or as) the entity satisfies a performance obligation. The guidance will significantly enhance comparability of revenue recognition practices across entities, industries, jurisdictions and capital markets. Additionally, the guidance requires improved disclosures as to the nature, amount, timing and uncertainty of revenue that is recognized. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarified the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarified the implementation guidance regarding performance obligations and licensing arrangements. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606)—Narrow-Scope Improvements and Practical Expedients, which clarified guidance on assessing collectability, presenting sales tax, measuring noncash consideration, and certain transition matters. In December 2016, the FASB issued ASU No. 2016-20, Revenue from Contracts with Customers (Topic 606)—Technical Corrections and Improvements, which provided disclosure relief, and clarified the scope and application of the new revenue standard and related cost guidance. The guidance is effective for the annual reporting period beginning after December 15, 2017, including interim periods within that reporting period. Substantially all of our income is not within the scope of ASU 2014-09. For those income items that are within the scope (primarily fee income), we have similar performance obligations as compared with deliverables and separate units of account previously identified. As a result, our timing of income recognition remains the same and the adoption of the standard was not material.

        In February 2016, the FASB issued ASU 2016-02, Leases, which requires lessees to recognize on the balance sheet a right-of-use asset, representing its right to use the underlying asset for the lease term, and a lease liability for all leases with terms greater than 12 months. The guidance also requires qualitative and quantitative disclosures designed to assess the amount, timing, and uncertainty of cash flows arising from leases. The standard requires the use of a modified retrospective transition approach, which includes a number of optional practical expedients that entities may elect to apply. The guidance is effective for annual periods beginning after December 15, 2018, and interim periods therein. Early application is permitted. While we continue to assess the effect of adoption, we currently believe the most significant change relates to the recognition of a new right-of-use asset and lease liability on our consolidated balance sheet for our office space operating lease. We currently have one operating lease for office space and do not expect a significant change in our leasing activity between now and adoption. See further discussion of our operating lease obligation in "Note K—Commitments and Contingences" in the notes to the consolidated financial statements.

        In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230), which is intended to reduce the existing diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The guidance is effective for annual periods beginning after December 15, 2017, and interim periods therein. We have adopted ASU 2016-15. The impact of the adoption of this accounting standard on our consolidated financial statements was not material.

        In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820), which is intended to improve fair value and defined benefit disclosure requirements by removing disclosures that are not cost-beneficial, clarifying disclosures' specific requirements, and adding relevant disclosure requirements. The amendments take effect for all organizations for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. We have

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elected to early adopt ASU 2018-13 in the current annual period. No significant changes were made to our fair value disclosures in the notes to the consolidated financial statements in order to comply with ASU 2018-13.

        In August 2018, the SEC adopted rules (the "SEC Release") amending certain disclosure requirements intended to eliminate redundant, duplicative, overlapping, outdated, or superseded, in light of other SEC disclosure requirements, US GAAP requirements, or changes in the information environment. In part, the SEC Release requires an investment company to present distributable earnings in total on the consolidated balance sheet and consolidated statement of changes in net assets, rather than showing the three components of distributable earnings as previously shown. We adopted this part of the SEC Release in the current annual period and the changes in presentation have been retrospectively applied to the consolidated balance sheet as of December 31, 2017 and to the consolidated statements of changes in net assets for the years ended December 31, 2017 and 2016. The impact of the adoption of these rules on our consolidated financial statements was not material. Additionally, the SEC Release requires disclosure of changes in net assets within a registrant's Form 10-Q filing on a quarter-to-date and year-to-date basis for both the current year and prior year comparative periods. We expect to adopt the new requirement to present changes in shareholders' equity in interim financial statements within Form 10-Q filings starting with the quarter ending March 31, 2019. The compliance date for the SEC Release was for all filings, as applicable, on or after November 5, 2018. The adoption of these additional rules will not have a material impact on the consolidated financial statements.

        From time to time, new accounting pronouncements are issued by the FASB or other standards setting bodies that are adopted by us as of the specified effective date. We believe that the impact of recently issued standards and any that are not yet effective will not have a material impact on our consolidated financial statements upon adoption.

Inflation

        Inflation has not had a significant effect on our results of operations in any of the reporting periods presented herein. However, our portfolio companies have experienced, and may in the future experience, the impacts of inflation on their operating results, including periodic escalations in their costs for labor, raw materials and third-party services and required energy consumption.

Off-Balance Sheet Arrangements

        We may be a party to financial instruments with off-balance sheet risk in the normal course of business to meet the financial needs of our portfolio companies. These instruments include commitments to extend credit and fund equity capital and involve, to varying degrees, elements of liquidity and credit risk in excess of the amount recognized in the balance sheet. At December 31, 2018, we had a total of $136.9 million in outstanding commitments comprised of (i) 33 investments with commitments to fund revolving loans that had not been fully drawn or term loans with additional commitments not yet funded and (ii) 11 investments with equity capital commitments that had not been fully called.

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Contractual Obligations

        As of December 31, 2018, the future fixed commitments for cash payments in connection with our SBIC debentures, the 4.50% Notes due 2019, the 4.50% Notes due 2022 and rent obligations under our office lease for each of the next five years and thereafter are as follows:

        As of December 31, 2018, we had $301.0 million in borrowings outstanding under our Credit Facility, and the Credit Facility is currently scheduled to mature in September 2023. The Credit Facility contains two, one-year extension options which could extend the maturity to September 2025, subject to lender approval. See further discussion of the Credit Facility terms in "—Liquidity and Capital Resources—Capital Resources."

Related Party Transactions

        As discussed further above, the External Investment Manager is treated as a wholly owned portfolio company of MSCC and is included as part of our Investment Portfolio. At December 31, 2018, we had a receivable of approximately $2.9 million due from the External Investment Manager which included approximately $1.8 million primarily related to operating expenses incurred by us as required to support the External Investment Manager's business and amounts due from the External Investment Manager to Main Street under a tax sharing agreement (see further discussion above in "—Critical Accounting Policies—Income Taxes") and approximately $1.2 million of dividends declared but not paid by the External Investment Manager.

        In November 2015, our Board of Directors approved and adopted the Main Street Capital Corporation Deferred Compensation Plan (the "2015 Deferred Compensation Plan"). The 2015 Deferred Compensation Plan became effective on January 1, 2016 and replaced the Deferred Compensation Plan for Non-Employee Directors previously adopted by the Board of Directors in June 2013 (the "2013 Deferred Compensation Plan"). Under the 2015 Deferred Compensation Plan, non-employee directors and certain key employees may defer receipt of some or all of their cash compensation and directors' fees, subject to certain limitations. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan and are also able to direct investments made on their behalf among investment alternatives permitted from time to time under the plan, including phantom Main Street stock units. As of December 31, 2018, $6.1 million of compensation and directors' fees had been deferred under the 2015 Deferred Compensation Plan (including amounts previously deferred under the 2013 Deferred Compensation Plan). Of this amount, $3.3 million was deferred into phantom Main Street stock units, representing 97,344 shares of our common stock.

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Including phantom stock units issued through dividend reinvestment and net of any shares distributed, the phantom stock units outstanding as of December 31, 2018 represented 119,639 shares of our common stock. Any amounts deferred under the plan represented by phantom Main Street stock units will not be issued or included as outstanding on the consolidated statements of changes in net assets until such shares are actually distributed to the participant in accordance with the plan, but are included in operating expenses and weighted-average shares outstanding in our consolidated statements of operations as earned.

Recent Developments

        In January 2019, we led a new portfolio investment to facilitate the minority recapitalization of Centre Technologies, Inc. ("Centre"), a premier provider of IT hardware, software and service solutions. We, along with our co-investors, partnered with Centre's founder and Chief Executive Officer and management team to facilitate the transaction, with us funding $18.1 million in a combination of first-lien, senior secured term debt and a direct equity investment. Headquartered in Houston, Texas, and founded in 2006, Centre has established itself as a mission critical IT solutions provider offering a full suite of solutions including managed and hosted services, value-added sourcing and integration, and project services.

        In January 2019, we led a new portfolio investment to facilitate the management buyout of CompareNetworks Inc. ("CompareNetworks"), a leading provider of media, marketing and technology solutions that drive revenue for life science and healthcare product manufacturers. We, along with our co-investors, partnered with CompareNetworks' founders and management team to facilitate the transaction, with us funding $10.7 million in a combination of first-lien, senior secured term debt and a direct equity investment. Headquartered in South San Francisco, California, and founded in 2000, CompareNetworks provides life scientists, researchers, lab-based professionals, pharmaceutical professionals and healthcare professionals with digital tools and information resources to research, identify and determine which products and technologies to use.

        In January 2019, we fully exited our equity investment in Boss Industries, LLC ("Boss"). Boss markets, designs and manufacturers vehicle-mounted, and portable air compressor and generator systems utilized in municipal and utility services, energy product and industrial services. We realized a gain of approximately $4.0 million on the exit of our equity investment in Boss.

        During February 2019, we declared regular monthly dividends of $0.200 per share for each month of April, May and June 2019. These regular monthly dividends equal a total of $0.600 per share for the second quarter of 2019 and represent a 5.3% increase from the dividends declared for the second quarter of 2018. Including the dividends declared for the second quarter of 2019, we will have paid $25.420 per share in cumulative dividends since our October 2007 initial public offering.

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MANAGEMENT

        Our business and affairs are managed under the direction of our Board of Directors. Our Board of Directors appoints our officers, who serve at the discretion of the Board of Directors. The responsibilities of the Board of Directors include, among other things, the oversight of our investment activities, the quarterly valuation of our assets, oversight of our financing arrangements and corporate governance activities. The Board of Directors has an Audit Committee, Compensation Committee, and Nominating and Corporate Governance Committee, and may establish additional committees from time to time as necessary.

Board of Directors and Executive Officers

        Our Board of Directors consists of ten members, eight of whom are classified under applicable NYSE listing standards as "independent" directors and under Section 2(a)(19) of the 1940 Act as not "interested persons." Pursuant to our articles of incorporation, each member of our Board of Directors serves a one-year term, with each current director serving until the 2019 annual meeting of stockholders and until his or her respective successor is duly qualified and elected. Our articles of incorporation give our Board of Directors sole authority to appoint directors to fill vacancies that are created either through an increase in the number of directors or due to the resignation, removal or death of any director.

Directors

        Information regarding our current Board of Directors is set forth below as of March 6, 2019. We have divided the directors into two groups—independent directors and interested directors. Interested directors are "interested persons" of MSCC as defined in Section 2(a)(19) of the 1940 Act. The address for each director is c/o Main Street Capital Corporation, 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056.

Independent Directors

Interested Directors

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Executive Officers

        Our executive officers serve at the discretion of our Board of Directors. The following persons serve as our executive officers or significant employees in the following capacities (information as of March 6, 2019):

        The address for each executive officer and significant employee is c/o Main Street Capital Corporation, 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056.

Biographical Information

Independent Directors

        Michael Appling, Jr. is the Chief Executive Officer of TNT Crane & Rigging Inc., a privately held full service crane and rigging operator. From July 2002 through August 2007, he was the Executive Vice President and Chief Financial Officer of XServ, Inc., a large private equity funded, international industrial services and rental company. Mr. Appling also held the position of CEO and President for United Scaffolding, Inc., an XServ, Inc. operating subsidiary. In February 2007, XServ, Inc. was sold to The Brock Group, a private industrial services company headquartered in Texas. From March 2000 to June 2002, Mr. Appling served as the Chief Financial Officer of CheMatch.com, an online commodities trading forum. ChemConnect, Inc., a venture backed independent trading exchange, acquired CheMatch.com in January 2002. From June 1999 to March 2000, Mr. Appling was Vice President and Chief Financial Officer of American Eco Corporation, a publicly traded, international fabrication, construction and maintenance provider to the energy, pulp and paper and power industries. Mr. Appling worked for ITEQ, Inc., a publicly traded, international fabrication and services company,

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from September 1997 to May 1999, first as a Director of Corporate Development and then as Vice President, Finance and Accounting. From July 1991 to September 1997, Mr. Appling worked at Arthur Andersen, where he practiced as a certified public accountant. We believe Mr. Appling is qualified to serve on our Board of Directors because of his extensive finance and accounting experience, as well as his executive leadership and management experience as a chief executive officer.

        Valerie L. Banner has served as Senior Vice President, General Counsel and Corporate Secretary of Exterran Corporation (NYSE: EXTN), a global systems and process company offering solutions in the oil, gas, water and power markets, since November 2018, and as its Vice President, General Counsel and Corporate Secretary from November 2015 through October 2018. Prior to the spin-off of Exterran Corporation from Archrock, Inc., formerly known as Exterran Holdings, Inc. (NYSE: AROC, formerly EXH) in November 2015, Ms. Banner served as Associate General Counsel of Exterran Holdings from 2008 to 2015 and as special counsel from 2007 to 2008. Prior to the merger of Hanover Compressor Company and Universal Compression Holdings, Inc. in August 2007 to form Exterran Holdings, she served Universal as special counsel from 2000 to 2007, and served as Senior Vice President, General Counsel and Secretary from 1998 through 2000. Ms. Banner held various roles prior to 1998, including Vice President, General Counsel and Secretary of Team, Inc. (NYSE: TISI), having begun her career as an associate with Hunton Andrews Kurth LLP. Ms. Banner also serves as an officer and director of certain Exterran Corporation domestic and international subsidiaries. We believe Ms. Banner is qualified to serve on our Board of Directors because of her extensive legal and executive leadership experience at public companies and her broad knowledge in the areas of mergers and acquisitions, corporate finance, risk management, regulatory compliance and corporate governance.

        Joseph E. Canon, since 2008, Mr. Canon has been the Executive Vice President and Executive Director, and a member of the Board of Directors, of Kickapoo Springs Foundation and The Legett Foundation, two private family foundations, as well as the General Manager of a family office, all located in Abilene, Texas. He has also been involved during this time as an executive officer and director of several private companies and partnerships with emphasis on energy, financial and other alternative investments. From 1982 to December 2018, Mr. Canon served as the Executive Vice President and Executive Director, and a member of the Board of Directors, of Dodge Jones Foundation, a private charitable foundation located in Abilene, Texas. From 1974 to 1982, he served as Executive Vice President and Trust Officer of First National Bank of Abilene. Mr. Canon served until April 2014 on the Board of Directors of First Financial Bankshares, Inc. (NASDAQ: FFIN), a bank and financial holding company headquartered in Abilene, Texas. Mr. Canon also served until April 2014 on the Board of Directors for several bank and trust/asset management subsidiaries of First Financial Bankshares, Inc. and currently serves on the Investment Committee of First Financial Trust and Asset Management Company, a $5.6 billion trust services company affiliated with FFIN. He has also served as an executive officer and member of the Board of Directors of various other organizations including the Abilene Convention and Visitors Bureau, Abilene Chamber of Commerce, Conference of Southwest Foundations, City of Abilene Tax Increment District, West Central Texas Municipal Water District and the John G. and Marie Stella Kenedy Memorial Foundation. We believe Mr. Canon's qualifications to serve on our Board of Directors include his many years of managing and investing assets on behalf of public and private entities, his considerable experience in trust banking activities and practices, and his experience on other public boards of directors.

        Arthur L. French has served in a variety of executive management and board of director roles over the course of his business career. He began his private investment activities in 2000 and served as a director of Fab Tech Industries, a steel fabricator, from November 2000 until August 2009, as a director of Houston Plating and Coatings Company, an industrial coatings company, from 2002 until 2007, as a director of Rawson LP, an industrial distribution and maintenance services company, from May 2003 until June 2009, and as non-executive chairman of Rawson Holdings, LLC from March 2009 until December 2010. From September 2003 through March 2007, Mr. French was a member of the Advisory

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Board of Main Street Capital Partners, LLC and a limited partner of Main Street Mezzanine Fund, LP (both of which are now subsidiaries of Main Street). Mr. French currently serves as an advisor to LKCM Capital Group, LLC, an alternative investment vehicle for Luther King Capital Management headquartered in Fort Worth, Texas ("LKCM"). In addition, he serves as an independent director, Chairman of the Audit Committee and a member of the Compensation Committee of Relevant Solutions Inc. (previously LKCM Distribution Holdings LP), an LKCM portfolio company which provides industrial instrumentation and controls, air compressor products and systems, heat transfer and filter systems and related maintenance services to chemical, petrochemical, oil and gas and power generation customers in Texas, Oklahoma and Louisiana as well as other key markets in the central and western United States. From 1996 to 1999, Mr. French was Chairman and Chief Executive Officer of Metals USA Inc. (NYSE), where he managed the process of founders acquisition, assembled the management team and took the company through a successful IPO in July 1997. From 1989 to 1996, he served as Executive Vice President and Director of Keystone International, Inc. (NYSE), a manufacturer of flow controls equipment. After serving as a helicopter pilot in the United States Army, Captain, Corps of Engineers from 1963 to 1966, Mr. French began his career as a Sales Engineer for Fisher Controls International, Inc., in 1966. During his 23-year career at Fisher Controls, from 1966 to 1989, Mr. French held various titles, and ended his career at Fisher Controls as President, Chief Operating Officer and Director. We believe Mr. French is qualified to serve on our Board of Directors because of his executive management and leadership roles within numerous public and private companies and his experience in investing in private companies.

        J. Kevin Griffin is the Senior Vice President of Financial Planning & Analysis at Novant Health, a not-for-profit integrated system of 15 hospitals and a medical group consisting of approximately 1,600 physicians in 500 clinic locations, as well as numerous outpatient surgery centers, medical plazas, rehabilitation programs, diagnostic imaging centers, and community health outreach programs. Mr. Griffin's responsibilities at Novant primarily include debt capital market and M&A transactions, along with various other strategic analysis projects. From 2007 to October 2012, Mr. Griffin was a Managing Director of Fennebresque & Co., LLC, a boutique investment banking firm located in Charlotte, North Carolina. From 2003 through 2007, he was a Partner at McColl Partners, LLC, where he originated and executed middle market M&A transactions. Prior to McColl Partners, Mr. Griffin worked in the M&A and corporate finance divisions of Lazard Ltd, JPMorgan, and Bank of America in New York, Chicago, and Charlotte. Mr. Griffin's investment banking experience consists primarily of executing and originating mergers and acquisitions and corporate finance transactions. We believe Mr. Griffin is qualified to serve on our Board of Directors because of his extensive finance and valuation experience, his knowledge of the healthcare industry, and his extensive background in working with middle market companies in an M&A and advisory capacity.

        John E. Jackson is the President and Chief Executive Officer of Spartan Energy Partners, LP, a gas gathering, treating and processing company. He has also been a director of Seitel, Inc., a privately owned provider of onshore seismic data to the oil and gas industry in North America, since August 2007, CNX Midstream Partners, LP, formerly known as CONE Midstream Partners, LP (NYSE: CNXM, formerly CNNX), a master limited partnership that owns and operates natural gas gathering and other midstream energy assets in the Marcellus Shale in Pennsylvania and West Virginia, since January 2015, and Basic Energy Services, Inc. (NYSE: BAS), a provider of well site services in the United States to oil and natural gas drilling and producing companies, since December 2016. Mr. Jackson was Chairman, Chief Executive Officer and President of Price Gregory Services, Inc., a pipeline-related infrastructure service provider in North America, from February 2008 until its sale in October of 2009. He served as a director of Hanover Compressor Company ("Hanover"), now known as Exterran Corporation (NYSE: EXTN) and Archrock, Inc. (NYSE: AROC), from July 2004 until May 2010. Mr. Jackson also served as Hanover's President and Chief Executive Officer from October 2004 to August 2007 and as Chief Financial Officer from January 2002 to October 2004. He also serves on the board of several non-profit organizations. We believe Mr. Jackson's qualifications to serve on

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our Board of Directors include his extensive background in executive and director roles of public and private companies.

        Brian E. Lane has served as Chief Executive Officer and President of Comfort Systems USA, Inc. (NYSE: FIX), a leading provider of commercial, industrial and institutional heating, ventilation and air conditioning ("HVAC") services, since December 2011 and as a director of Comfort Systems since November 2010. Mr. Lane served as Comfort Systems' President and Chief Operating Officer from March 2010 until December 2011. Mr. Lane joined Comfort Systems in October 2003 and served as Vice President and then Senior Vice President for Region One until he was named Executive Vice President and Chief Operating Officer in January 2009. Prior to joining Comfort Systems, Mr. Lane spent fifteen years at Halliburton Company (NYSE: HAL), a global service and equipment company devoted to energy, industrial, and government customers. During his tenure at Halliburton, he held various positions in business development, strategy and project initiatives, and he departed as the Regional Director of Europe and Africa. Mr. Lane's additional experience included serving as a Regional Director of Capstone Turbine Corporation (NASDAQ: CPST), a distributed power manufacturer. He also was a Vice President of Kvaerner, an international engineering and construction company, where he focused on the chemical industry. Mr. Lane is also a member of the Board of Directors of Griffen Dewatering Corporation, a privately held company. Mr. Lane earned a Bachelor of Science in Chemistry from the University of Notre Dame and his MBA from Boston College. We believe Mr. Lane is qualified to serve on our Board of Directors because of his background in executive and director roles of public and private companies and his extensive knowledge of the construction and industrial services industries.

        Stephen B. Solcher has served as the Senior Vice President of Finance and Business Operations and Chief Financial Officer of BMC Software, Inc., a privately held company that is a global leader in software solutions, since 2005. Previously, Mr. Solcher served as BMC's Treasurer and Vice President of Finance. He joined BMC in 1991 as Assistant Treasurer and became Treasurer the following year. During Mr. Solcher's tenure, BMC grew from nearly $130 million in annual revenue to $2.2 billion in annual revenue in 2013, its last year operating as a public company. In addition to leading many M&A transactions as Chief Financial Officer, Mr. Solcher was instrumental in BMC's transition from being a publicly traded company to becoming a privately held company in 2013. Prior to joining BMC, he was employed by Arthur Andersen as a certified public accountant. Mr. Solcher also serves on the development board of the Mays Business School at Texas A&M University and has served on the board of numerous nonprofit organizations. He was recognized by Institutional Investor magazine as part of the "All American Executive Team" in 2010 and 2012 and by Houston Business Journal as 2012 Best CFO—Large Public Company. We believe Mr. Solcher's qualifications to serve on our Board of Directors include his thorough knowledge of the information technology and software industries and his accounting, finance and M&A experience as a chief financial officer of a large public and private company qualifying him to be an audit committee financial expert.

Interested Directors

        Vincent D. Foster has served as Executive Chairman since November 2018 and as Chairman of Main Street's Board of Directors since 2007. Mr. Foster previously served as Main Street's Chief Executive Officer from 2007 until November 2018 and also served as Main Street's President from 2012 until 2015. He has also been a member of our management team's investment committee since its formation in 2007, a member of our management team's credit committee since its formation in 2011 and a member of our management team's executive committee since its formation in 2015. Mr. Foster also currently serves as a founding director of Quanta Services, Inc. (NYSE: PWR), which provides specialty contracting services to the power, natural gas and telecommunications industries. He also served as a director of U.S. Concrete, Inc. (NASDAQ-CM: USCR) from 1999 until 2010, Carriage Services, Inc. (NYSE: CSV) from 1999 to 2011, HMS Income Fund, Inc., a non-publicly traded

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business development company of which MSC Adviser I, LLC, a wholly owned subsidiary of Main Street, acts as the investment sub-adviser, from 2012 until 2013 and Team, Inc. (NYSE: TISI) from 2005 until 2017. In addition, Mr. Foster served as a founding director of the Texas TriCities Chapter of the National Association of Corporate Directors from 2004 to 2011. Mr. Foster, a certified public accountant, had a 19-year career with Arthur Andersen, where he was a partner from 1988 to 1997. Mr. Foster was the director of Arthur Andersen's Corporate Finance and Mergers and Acquisitions practice for the Southwest United States and specialized in working with companies involved in consolidating their respective industries. From 1997, Mr. Foster co-founded and has acted as co-managing partner and in other senior executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours. We believe Mr. Foster is qualified to serve on our Board of Directors because of his intimate knowledge of our business and operations, along with his comprehensive experience on other public Boards of Directors and his extensive experience in tax, accounting, mergers and acquisitions, corporate governance and finance.

        Dwayne L. Hyzak has served as Main Street's Chief Executive Officer since November 2018 and as a member of our Board of Directors since January 2018. Mr. Hyzak also serves as a member of our management team's investment, credit and executive committees. Previously, he served as President (2015 until November 2018), Chief Operating Officer (2014 until November 2018), Chief Financial Officer (2011 until 2014) and Senior Managing Director since 2011. Mr. Hyzak has served in other senior executive positions at Main Street since prior to its IPO in 2007. From 2002, Mr. Hyzak has also served as a Senior Managing Director and in other executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours. From 2000 to 2002, Mr. Hyzak was a Director of Integration with Quanta Services, Inc. (NYSE: PWR), which provides specialty contracting services to the power, natural gas and telecommunications industries, where he was principally focused on the company's mergers and acquisitions and corporate finance activities. Prior to joining Quanta Services, Inc., Mr. Hyzak, a certified public accountant, was a Manager with Arthur Andersen in its Transaction Advisory Services group. We believe Mr. Hyzak is qualified to serve on our Board of Directors because of his long tenure in leadership roles at Main Street, currently as Chief Executive Officer and previously as President and Chief Operating Officer, in which roles he has successfully led our lower middle market investment activities, and also previously as Chief Financial Officer, along with his extensive experience in investing and managing investments in lower middle market companies, mergers and acquisitions, corporate finance, tax and accounting.

Non-Director Officers

        David L. Magdol has served as President since November 2018 and Chief Investment Officer and Senior Managing Director since 2011. Mr. Magdol is also the chairman of our management team's investment committee and a member of our management team's executive committee. Previously, he served as Vice Chairman from 2015 until November 2018. Mr. Magdol has served in other senior executive positions at Main Street since prior to its IPO in 2007. Mr. Magdol has also served as a Senior Managing Director and in other executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours, since 2002. Mr. Magdol joined Main Street from the investment banking group at Lazard Freres & Co. Prior to Lazard, he managed a portfolio of private equity investments for the McMullen Group, a private investment firm/family office capitalized by Dr. John J. McMullen, the former owner of the New Jersey Devils and the Houston Astros. Mr. Magdol began his career in the structured finance services group of JP Morgan Chase.

        Curtis L. Hartman has served as Vice Chairman since 2015 and Chief Credit Officer and Senior Managing Director since 2011. Mr. Hartman is also the chairman of our management team's credit committee and a member of our management team's investment and executive committees. Previously, he served in other senior executive positions at Main Street since prior to its IPO in 2007. From 2000, Mr. Hartman has also served as a Senior Managing Director and in other executive positions of several

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Main Street predecessor funds and entities, which are now subsidiaries of ours. Mr. Hartman also served on the Board of Directors of HMS Income Fund, Inc., a non-publicly traded business development company of which MSC Adviser I, LLC, a wholly owned subsidiary of Main Street, acts as the investment sub-adviser, from 2013 to 2016. Mr. Hartman currently serves as an executive officer of the Small Business Investor Alliance (SBIA) and has been a member of SBIA's Board of Governors since 2011 where he previously chaired the BDC Committee. From 1999 to 2000, Mr. Hartman was a Director for Sterling City Capital, LLC, a private investment firm. Concurrently with joining Sterling City Capital, he joined United Glass Corporation, a Sterling City Capital portfolio company, as Director of Corporate Development. Prior to joining Sterling City Capital, Mr. Hartman, a certified public accountant, was a manager with PricewaterhouseCoopers LLP in its M&A/Transaction Services group and a senior auditor with Deloitte & Touche LLP in its Financial Assurance Group.

        Jason B. Beauvais has served as Senior Vice President, General Counsel, Chief Compliance Officer and Secretary since 2012. Previously, Mr. Beauvais served as Vice President, General Counsel and Secretary since 2008. From 2006 through 2008, Mr. Beauvais was an attorney with Occidental Petroleum Corporation (NYSE: OXY), an international oil and gas exploration and production company. Prior to joining Occidental Petroleum Corporation, Mr. Beauvais practiced corporate and securities law at Baker Botts L.L.P., where he primarily counseled companies in public issuances and private placements of debt and equity and handled a wide range of general corporate and securities matters as well as mergers and acquisitions.

        Brent D. Smith has served as Chief Financial Officer and Treasurer since November 2014 and previously as Senior Vice President—Finance since August 2014. Mr. Smith previously served as Executive Vice President, Chief Financial Officer and Treasurer of Cal Dive International, Inc. from 2010 through June 2014 and in various finance and accounting roles at Cal Dive from 2005 through 2010. On March 3, 2015, Cal Dive and certain of its subsidiaries, excluding its foreign subsidiaries, filed for voluntary protection under Chapter 11 of the Bankruptcy Code. Prior to joining Cal Dive, Mr. Smith was a manager with FTI Consulting (NYSE: FCN). Prior to that, Mr. Smith, a certified public accountant, was employed as a senior auditor at Arthur Andersen LLP.

        Nicholas T. Meserve has served as Managing Director on our middle market investment team since 2012. Mr. Meserve is also a member of our management team's credit committee. Mr. Meserve has also served on the Board of Directors of HMS Income Fund, Inc., a non-publicly traded business development company of which MSC Advisor I, LLC, a wholly owned subsidiary of Main Street, acts as the investment sub-advisor, since April 2016. Previously, from 2004 until 2012, Mr. Meserve worked at Highland Capital Management, LP, a large alternative credit manager, and certain of its affiliates, where he managed a portfolio of senior loans and high yield bonds across a diverse set of industries. Prior to Highland, he was a Credit Analyst at JP Morgan Chase & Co.

        K. Colton Braud, III has served as a Managing Director on our lower middle market team since January 2017 and has been with the firm in Associate to Director roles since 2012. Prior to joining Main Street, Mr. Braud spent two years as an Associate at Wellspring Capital Management, a middle market private equity firm based in New York. While at Wellspring, Mr. Braud's responsibilities included evaluating leveraged buyout opportunities, conducting due diligence across a wide array of industries and portfolio management. Prior to Wellspring, Mr. Braud served as an Analyst at J.P. Morgan Securities Inc. in its Financial Sponsor Group.

        Alejandro Capetillo has served as a Managing Director on our lower middle market team since October 2017 and has been with the firm in Analyst to Director roles since 2008. During his time with Main Street, Mr. Capetillo has been part of a lower middle market team that has closed over 30 transactions, including growth financings, dividend recapitalizations, management buyouts and control leveraged buyouts. In addition to having led a number of platform and add-on transactions in diverse industries on behalf of Main Street, Mr. Capetillo has been a member of the board of directors

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of several of Main Street's portfolio companies, assisting those companies with issues ranging from day-to-day operations to broader growth and exit strategies.

        Samuel A. Cashiola has served as a Managing Director since January 2019 and various other roles since joining Main Street in 2012. Prior to joining Main Street, Mr. Cashiola was an Associate Analyst on Morgan Keegan's Internet & e-Commerce Equity Research team, which focused on companies in the digital media and e-commerce segments, and an Associate at T. Rowe Price Associates. Mr. Cashiola holds a B.S. degree in Finance from Wake Forest University and a M.S. degree in Finance from the Owen Graduate School of Management at Vanderbilt University. He is a CFA charterholder and is a member of the CFA Society of Houston.

        Watt R. Matthews has served as a Managing Director since January 2019 and in various other roles since joining Main Street in 2010. He graduated from the University of Texas at Austin in May 2008 with a B.B.A. in Finance degree and a minor in Accounting, followed by post-graduate study in Ranch Management from Texas Christian University.

        Shannon D. Martin has served as Vice President, Chief Accounting Officer and Assistant Treasurer since 2012. From 2006 to 2012, Mr. Martin worked as an independent consultant and performed financial advisory services for several clients, including functioning as acting Chief Accounting Officer from 2008 to 2011 for EquaTerra, Inc. From 1999 to 2006, Mr. Martin was a director of accounting integration and audit with Quanta Services, Inc. (NYSE: PWR), which provides specialty contracting services to the power, natural gas and telecommunications industries, where he focused on the development of integrated accounting, business and information system processes and the company's acquisition and integration strategies. From 1992 to 1999, Mr. Martin, a certified public accountant, worked at Arthur Andersen as a manager in the Commercial Services group.

        Katherine S. Silva, a certified public accountant, has served as Vice President since 2015, with responsibility for managing Small Business Administration matters and several administrative functions, and Assistant Treasurer since 2010, with responsibility for managing day-to-day treasury activities. She also serves as special assistant to Mr. Foster. Ms. Silva has worked at Main Street since 2005 and holds a Bachelor of Arts in Journalism from the University of Georgia.

CORPORATE GOVERNANCE

        We maintain a corporate governance section on our website which contains copies of the charters for the committees of our Board of Directors. The corporate governance section may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website. The corporate governance section contains the following documents, which are available in print to any stockholder who requests a copy in writing to Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056:

Audit Committee Charter
Nominating and Corporate Governance Committee Charter
Compensation Committee Charter

        In addition, our Code of Business Conduct and Ethics and our Corporate Governance and Stock Ownership Guidelines may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website and are available in print to any stockholder who requests a copy in writing. Our Board of Directors adopted the Code of Business Conduct and Ethics in order to establish policies, guidelines and procedures that promote ethical practices and conduct by Main Street and all its employees, officers and directors. All officers, directors and employees of Main Street are responsible for maintaining the level of integrity and for complying with the policies contained in the Code of Business Conduct and Ethics. Each employee of Main Street is required to acknowledge that he or she has received, read and understands the Code of Business

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Conduct and Ethics and agrees to observe the policies and procedures contained therein at the time of hire and annually thereafter. We intend to disclose any substantive amendments to, or waivers from, our Code of Business Conduct and Ethics within four business days of the waiver or amendment through a posting on our website. The Corporate Governance and Stock Ownership Guidelines adopted by our Board of Directors establish our corporate governance principles and practices on a variety of topics, including the responsibilities, composition and functioning of the Board, responsibilities of management and interaction with the Board and stock ownership guidelines for management and Board members. The Nominating and Corporate Governance Committee of our Board assesses the Corporate Governance and Stock Ownership Guidelines periodically and makes recommendations to the Board on any changes to implement.

Director Independence

        Our Board of Directors currently consists of ten members, eight of whom are classified under applicable listing standards of the New York Stock Exchange as "independent" directors and under Section 2(a)(19) of the 1940 Act as not "interested persons." Based on these independence standards and the recommendation of the Nominating and Corporate Governance Committee, our Board of Directors has affirmatively determined that the following directors are independent:

Michael Appling Jr.
Valerie L. Banner
Joseph E. Canon
Arthur L. French
J. Kevin Griffin
John E. Jackson
Brian E. Lane
Stephen B. Solcher

        Our Board of Directors considered certain portfolio investments and other transactions in which our independent directors may have had a direct or indirect interest, including the transactions, if any, described under the heading "Certain Relationships and Related Party Transactions" in evaluating each director's independence under the 1940 Act and applicable listing standards of the New York Stock Exchange, and the Board of Directors determined that no such transaction would impact the ability of any director to exercise independent judgment or impair his or her independence.

Communications with the Board

        Stockholders or other interested persons may send written communications to the members of our Board of Directors, addressed to Board of Directors, c/o Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056. All communications received in this manner will be delivered to one or more members of our Board of Directors.

Board Leadership Structure

        Mr. Foster currently serves as the Executive Chairman of our Board of Directors and, prior to November 2018, served as our Chief Executive Officer. In November 2018, in connection with a previously announced long-term succession plan, the Board of Directors elected Mr. Hyzak as Chief Executive Officer of the Company, succeeding Mr. Foster who continues to serve as Main Street's Executive Chairman. Mr. Foster is an "interested person" under Section 2(a)(19) of the 1940 Act due to his current position with the Company as Executive Chairman. The Board believes that Mr. Foster is currently best situated to serve as Chairman of our Board of Directors given his history with the Company, his deep knowledge of the Company's business and his extensive experience in managing private debt and equity investments in lower middle market companies and debt investments in middle

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market companies. The Company's independent directors bring experience, oversight and expertise from outside the Company and industry, while Mr. Foster brings Company-specific and industry-specific experience and expertise. The Board believes that the current leadership structure with Mr. Foster serving as Executive Chairman of the Board promotes strategy development and execution while facilitating effective, timely communication between management and the Board and is optimum for effective corporate governance.

        Our Board of Directors has designated Arthur L. French as Lead Independent Director to preside over all executive sessions of non-management directors. In the Lead Independent Director's absence, the remaining non-management directors may appoint a presiding director by majority vote. The non-management directors meet in executive session without management on a regular basis. The Lead Independent Director also has the responsibility of consulting with management on Board and committee meeting agendas, acting as a liaison between management and the non-management directors, including maintaining frequent contact with the Executive Chairman and the Chief Executive Officer and facilitating collaboration and communication between the non-management directors and management. Stockholders or other interested persons may send written communications to Arthur L. French, addressed to Lead Independent Director, c/o Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056.

Board of Directors and its Committees

        Board of Directors.    Our Board of Directors met four times and acted by unanimous written consent twenty-six times during 2018. All directors attended 100% of the meetings of the Board of Directors and of the committees on which they served during 2018, and all ten directors attended the 2018 annual meeting of stockholders in person. Our Board of Directors expects each director to make a diligent effort to attend all Board and committee meetings, as well as each annual meeting of stockholders.

        Committees.    Our Board of Directors currently has, and appoints the members of, standing Audit, Compensation and Nominating and Corporate Governance Committees. Each of those committees is comprised entirely of independent directors and has a written charter approved by our Board of Directors. The current members of the committees are identified in the following table.

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        Audit Committee.    During the year ended December 31, 2018, the Audit Committee met four times. The Audit Committee is responsible for selecting, engaging and discharging our independent accountants, reviewing the plans, scope and results of the audit engagement with our independent accountants, approving professional services provided by our independent accountants (as well as the compensation for those services), reviewing the independence of our independent accountants and reviewing the adequacy of our internal control over financial reporting. In addition, the Audit Committee is responsible for assisting our Board of Directors with its review and approval of the determination of the fair value of our investments. Our Board of Directors has determined that each of Messrs. Appling, Griffin, Jackson and Solcher is an "Audit Committee financial expert" as defined by the Securities and Exchange Commission, or the SEC. For more information on the backgrounds of these directors, see their biographical information under "Biographical Information" above.

        Compensation Committee.    During the year ended December 31, 2018, the Compensation Committee met four times and acted by unanimous written consent five times. The Compensation Committee determines the compensation and related benefits for our executive officers including the amount of salary, bonus and stock-based compensation to be included in the compensation package for each of our executive officers. In addition, the Compensation Committee assists the Board of Directors in developing and evaluating the compensation of our non-management directors and evaluating succession planning with respect to the chief executive officer and other key executive positions. The Compensation Committee has the authority to engage the services of outside advisers, experts and others as it deems necessary to assist the committee in connection with its responsibilities. The actions of the Compensation Committee are generally reviewed and ratified by the entire Board of Directors, except the employee directors do not vote with respect to their compensation.

        Nominating and Corporate Governance Committee.    During the year ended December 31, 2018, the Nominating and Corporate Governance Committee met four times. The Nominating and Corporate Governance Committee is responsible for determining criteria for service on our Board of Directors, identifying, researching and recommending to the Board of Directors director nominees for election by our stockholders, selecting nominees to fill vacancies on our Board of Directors or a committee of the Board, developing and recommending to our Board of Directors any amendments to our corporate governance principles and overseeing the self-evaluation of our Board of Directors and its committees.

Board and Committee Evaluation Process

        Each year, our Board of Directors conducts a thorough self-assessment evaluation process. Detailed questionnaires solicit anonymous input from directors regarding the performance and effectiveness of the Board, Board committees, individual directors and interaction with management and provide an opportunity for Board members to identify areas for improvement. The Nominating and Corporate Governance Committee reviews the results and feedback from this process and makes recommendations for improvements as appropriate. The Board has successfully used this process to evaluate Board and Board committee effectiveness and identify opportunities to strengthen the Board.

Compensation Committee Interlocks and Insider Participation

        Each member of the Compensation Committee is independent for purposes of the applicable listing standards of the New York Stock Exchange. During the year ended December 31, 2018, no member of the Compensation Committee was an officer, former officer or employee of ours or had a relationship disclosable under "Certain Relationships and Related Party Transactions," except as disclosed therein. No interlocking relationship, as defined by the rules adopted by the SEC, existed during the year ended December 31, 2018 between any member of the Board of Directors or the Compensation Committee and an executive officer of Main Street.

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Director Nomination Process

        Our Nominating and Corporate Governance Committee has determined that a candidate for election to our Board of Directors must satisfy certain general criteria, including, among other things:

•

be an individual of the highest character and integrity and have an inquiring mind, vision, a willingness to ask hard questions and the ability to work professionally with others;



•

be free of any conflict of interest that would violate any applicable law or regulation or interfere with the proper performance of the responsibilities of a director;



•

be willing and able to devote sufficient time to the affairs of our Company and be diligent in fulfilling the responsibilities of a member of our Board of Directors and a member of any committee thereof (including: developing and maintaining sufficient knowledge of our Company and the specialty finance industry in general; reviewing and analyzing reports and other information important to responsibilities of our Board of Directors and any committee thereof; preparing for, attending and participating in meetings of our Board of Directors and meetings of any committee thereof; and satisfying appropriate orientation and continuing education guidelines); and



•

have the capacity and desire to represent the balanced, best interests of our stockholders as a whole and not primarily a special interest group or constituency.

        The Nominating and Corporate Governance Committee seeks to identify potential director candidates who will strengthen the Board of Directors and will contribute to the overall mix of general criteria identified above. In addition to the general criteria, the Nominating and Corporate Governance Committee considers specific criteria, such as particular skills, experiences (whether in business or in other areas such as public service, academia or scientific communities), areas of expertise, specific backgrounds, and other characteristics, that should be represented on the Board of Directors to enhance its effectiveness and the effectiveness of its committees. The Nominating and Corporate Governance Committee does not have a formal policy with respect to diversity; however, the Board and the Nominating and Corporate Governance Committee believe that it is essential that the Board members represent diverse experience and viewpoints and a diverse mix of the specific criteria above. The process of identifying potential director candidates includes establishing procedures for soliciting and reviewing potential nominees from directors and for advising those who suggest nominees of the outcome of such review. The Nominating and Corporate Governance Committee also has the authority to retain and terminate any search firm used to identify director candidates.

        Any stockholder may nominate one or more persons for election as one of our directors at an annual meeting of stockholders if the stockholder complies with the notice, information and consent provisions contained in our bylaws and any other applicable law, rule or regulation regarding director nominations. When submitting a nomination to our Company for consideration, a stockholder must provide certain information that would be required under applicable SEC rules, including the following minimum information for each director nominee: full name, age and address; number of any shares of our stock beneficially owned by the nominee, if any; the date such shares were acquired and the investment intent of such acquisition; whether such stockholder believes the nominee is an "interested person" of our Company, as defined in 1940 Act; and all other information required to be disclosed in solicitations of proxies for election of directors in an election contest or is otherwise required, including the nominee's written consent to being named in the proxy statement as a nominee and to serving as a director if elected. See "Stockholders' Proposals" in our proxy statement and the relevant provisions of our bylaws for other requirements of stockholder proposals.

        The Nominating and Corporate Governance Committee will consider candidates identified through the processes described above, and will evaluate each of them, including incumbents, based on the same criteria. The Nominating and Corporate Governance Committee also takes into account the

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contributions of incumbent directors as Board members and the benefits to us arising from their experience on our Board of Directors. Although the Nominating and Corporate Governance Committee will consider candidates identified by stockholders, the Nominating and Corporate Governance Committee may determine not to recommend those candidates to our Board of Directors, and our Board of Directors may determine not to nominate any candidates recommended by the Nominating and Corporate Governance Committee. No director nominee named in this prospectus supplement was nominated by stockholders.

Board's Role in the Oversight of Risk Management

        Our Board of Directors as a whole has responsibility for risk oversight, with reviews of certain areas being conducted by the relevant Board committees that report on their deliberations to the full Board. The oversight responsibility of the Board and its committees is enabled by management reporting processes that are designed to provide visibility to the Board about the identification, assessment and management of critical risks and management's risk mitigation strategies. Areas of focus include competitive, economic, operational, financial (accounting, credit, liquidity and tax), legal, regulatory, compliance and other risks. The Board and its committees oversee risks associated with their respective principal areas of focus, as summarized below. Committees meet in executive session with key management personnel regularly and with representatives of outside advisors as necessary.

COMPENSATION OF DIRECTORS

        The following table sets forth the compensation that we paid during the year ended December 31, 2018 to our non-employee directors. Directors who are also employees of Main Street or any of its subsidiaries do not receive compensation for their services as directors.

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Director Compensation Table

        The compensation for non-employee directors for 2018 was comprised of cash compensation paid to or earned by directors in connection with their service as a director. That cash compensation consisted of an annual retainer of $175,000, and an additional $42,500 retainer for the Lead

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Independent Director. Non-employee directors do not receive fees based on meetings attended absent circumstances that require an exceptionally high number of meetings within an annual period. We also reimburse our non-employee directors for all reasonable expenses incurred in connection with their service on our Board. The chairpersons and members of our Board committees received additional annual retainers for 2018 as follows:

•

the chairperson of the Audit Committee: $32,500;



•

members of the Audit Committee other than the chairperson: $15,000;



•

the chairperson of the Compensation Committee: $20,000;



•

members of the Compensation Committee other than the chairperson: $10,000;



•

the chairperson of the Nominating and Corporate Governance Committee: $15,000; and



•

members of the Nominating and Corporate Governance Committee other than the chairperson: $7,500.

        The Non-Employee Director Plan provides a means through which we may attract and retain qualified non-employee directors to enter into and remain in service on our Board of Directors. Under the Non-Employee Director Plan, at the beginning of each one-year term of service on our Board of Directors, each non-employee director receives a number of shares equivalent to $30,000 based on the closing price of a share of our common stock on the New York Stock Exchange (or other exchange on which our shares are then listed) on the date of grant. These shares are subject to forfeiture provisions that will lapse as to an entire award at the end of the one-year term.

        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan. The 2015 Deferred Compensation Plan became effective on January 1, 2016 and replaced the Deferred Compensation Plan for Non-Employee Directors previously adopted and approved by the Board in 2013. Under the 2015 Deferred Compensation Plan, non-employee directors and certain key employees may defer receipt of some or all of their cash compensation, subject to certain limitations. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan and are also able to direct investments made on their behalf among investment alternatives permitted from time to time under the plan, including phantom Main Street stock units for which shares of Main Street common stock will be issued upon distribution.

        For the beneficial ownership of our common stock by each of our directors and the dollar range value of such ownership, please see "Control Persons and Principal Stockholders".

COMPENSATION DISCUSSION AND ANALYSIS

        The following Compensation Discussion and Analysis, or CD&A, provides information relating to the compensation of Main Street's Named Executive Officers, or NEOs, for 2018, who were:

•

Dwayne L. Hyzak, Member of the Board, Chief Executive Officer and Senior Managing Director;



•

David L. Magdol, President, Chief Investment Officer and Senior Managing Director;



•

Vincent D. Foster, Executive Chairman of the Board;



•

Jason B. Beauvais, Senior Vice President, General Counsel, Chief Compliance Officer and Secretary;



•

Brent D. Smith, Chief Financial Officer and Treasurer; and



•

Nicholas T. Meserve, Managing Director.

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        In November 2018, Main Street announced that in connection with its long-term succession plans, Mr. Hyzak was promoted to Chief Executive Officer, succeeding Mr. Foster in that role. Mr. Foster continues to serve as Main Street's Executive Chairman. Main Street also announced the promotion of Mr. Magdol to President in November 2018, succeeding Mr. Hyzak in that role.

Compensation Philosophy and Objectives

        The Main Street compensation system was developed by the Compensation Committee and approved by all independent directors. The system is designed to attract and retain key executives, motivate them to achieve the Company's business objectives and reward them for performance while aligning management's interests with those of the Company's stockholders. The structure of Main Street's incentive compensation programs is formulated to encourage and reward the following, among other things:

•

achievement of income and capital gains to sustain and grow the Company's dividend payments;



•

maintenance of liquidity and capital flexibility to accomplish the Company's business objectives, including the preservation of investor capital;



•

attainment of superior risk-adjusted returns on the Company's investment portfolio; and



•

professional development and growth of individual executives, the management team and other employees.

        The Compensation Committee has the primary authority to establish compensation for the NEOs and other key employees and administers all executive compensation arrangements and policies. Main Street's Chief Executive Officer assists the Committee by providing recommendations regarding the compensation of NEOs and other key employees, excluding himself. The Committee exercises its discretion by modifying or accepting these recommendations. The Chief Executive Officer routinely attends a portion of the Committee meetings. However, the Committee often meets in executive session without the Chief Executive Officer or other members of management when discussing compensation matters and on other occasions as determined by the Committee.

        The compensation packages for Main Street NEOs and other key employees are structured to reflect the Compensation Committee's commitment to corporate governance best practices and performance-oriented executive compensation. Specifically, the Compensation Committee has implemented the following practices for NEOs and other key employees:

•

no employment agreements;



•

no cash severance benefits;



•

no supplemental defined benefit pensions; and



•

no tax gross-up payments.

        The Compensation Committee believes the above practices are appropriate in light of the Company's current objectives and compensation philosophies but reserves the right to re-visit these practices in the future as may be appropriate. The Compensation Committee takes into account competitive market practices with respect to the salaries and total direct compensation of the NEOs and other key employees. Members of the Committee consider market practices by reviewing public and non-public information for executives at comparable companies and funds. The Committee also has the authority to utilize compensation consultants to better understand competitive pay practices and has retained such expertise in the past.

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Independent Compensation Consultant

        The Compensation Committee has from time to time engaged independent compensation consultants to assist the Committee and provide advice on a variety of compensation matters relating to NEO, other key employee and independent director compensation, incentive compensation plans and compensation trends, best practices and regulatory matters. Any such compensation consultants are hired by and report directly to the Compensation Committee. Although compensation consultants may work directly with management on behalf of the Compensation Committee, any such work is under the control and supervision of the Compensation Committee. The Compensation Committee did not retain any independent compensation consultants or pay any fees for compensation consulting services for fiscal 2018.

Assessment of Market Data

        In assessing the competitiveness of executive compensation levels, the Compensation Committee analyzes market data of certain companies, including internally managed business development companies, or BDCs, private equity firms and other asset management and financial services companies. This analysis focuses on key elements of compensation practices in general, and more specifically, the compensation practices at companies and funds reasonably comparable in asset size, typical investment size and type, market capitalization and general business scope as compared to the Company.

        As regards other internally managed BDCs like Main Street, the Compensation Committee considers the compensation practices and policies pertaining to executive officers as detailed in their company's respective proxies, research analysts' reports and other publicly available information. However, there are relatively few internally managed BDCs and none that are directly comparable to the Company as regards business strategies, assets under management, typical investment size and type and market capitalization. Moreover, regarding the compensation and retention of executive talent, the Company also competes with private equity funds, private credit funds, hedge funds and other types of specialized investment funds. Since these funds are generally private companies that are not required to publicly disclose their executive compensation practices and policies, the Committee relies on third-party compensation surveys as well as other available information to compare compensation practices and policies.

        Items taken into account from comparable companies and funds include, but are not necessarily limited to, base compensation, bonus compensation, stock option awards, restricted stock awards, carried interest and other compensation. In addition to actual levels of cash and equity related compensation, the Compensation Committee also considers other approaches comparable companies are taking with regard to overall executive compensation practices. Such items include, but are not necessarily limited to, the use of employment agreements for certain employees, the mix of cash and equity compensation, the use of third-party compensation consultants and certain corporate and executive performance measures that are established to achieve longer term total return for stockholders. Finally, in addition to analyzing comparable companies and funds, the Committee also evaluates the relative cost structure of the Company as compared to the entire BDC sector, including internally and externally managed BDCs, as well as other private funds.

Assessment of Company Performance

        The Compensation Committee believes that sustained financial performance coupled with consistent stockholders' returns as well as proportional employee compensation are essential components for Main Street's long-term business success. Main Street typically makes three to seven-year investments in its portfolio companies. However, the Company's business plan involves taking on investment risks over a range of time periods. Accordingly, much emphasis is focused on

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maintaining the stability of net asset values as well as the continuity of earnings to pass through to stockholders in the form of recurring dividends. The quality of the earnings supporting the dividends as well as the maintenance and growth of dividends are key metrics in the Committee's assessment of financial performance.

        Main Street's primary strategy is to generate current income from debt investments and to realize capital gains from equity-related investments. This income supports the payment of dividends to stockholders. The recurring payment of dividends requires a methodical investment acquisition approach and active monitoring and management of the investment portfolio over time. A meaningful part of the Company's employee base is dedicated to the maintenance of asset values and expansion of this recurring income to sustain and grow dividends. The Committee believes that stability of the management team is critical to achieving successful implementation of the Company's strategies. Further, the Committee, in establishing and assessing executive salary and performance incentives, is more focused on Main Street results as compared to its business objectives rather than the performance of Main Street relative to other comparable companies or industry metrics.

Executive Compensation Components

        For 2018, the components of Main Street's direct compensation program for NEOs included:

•

base salary;



•

annual cash bonuses;



•

long-term compensation pursuant to the 2015 Equity and Incentive Plan; and



•

other benefits.

        The Compensation Committee designs each NEO's direct compensation package to appropriately reward the NEO for his or her contribution to the Company. The judgment and experience of the Committee are weighed with individual and Company performance metrics and consultation with the Chief Executive Officer (except with respect to himself) to determine the appropriate mix of compensation for each individual. The Compensation Committee does not target a specific level of compensation relative to market practice, and only uses such data as a reference point when establishing compensation levels for NEOs. Cash compensation consisting of base salary and discretionary bonuses tied to achievement of individual performance goals that are reviewed and approved by the Committee, as well as corporate objectives, are intended to motivate NEOs to remain with the Company and work to achieve expected business objectives. Stock-based compensation is awarded based on performance expectations approved by the Committee for each NEO. The blend of short-term and long-term compensation may be adjusted from time to time to balance the Committee's views regarding the benefits of current cash compensation and appropriate retention incentives.

Base Salary

        Base salary is used to recognize the experience, skills, knowledge and responsibilities required of the NEOs in their roles. In connection with establishing the base salary of each NEO, the Compensation Committee and management consider a number of factors, including the seniority and experience level of the individual, the functional responsibilities of the position, the experience level of the individual, the Company's ability to replace the executive, the past base salary of the individual and the relative number of well-qualified candidates available in the area. In addition, the Committee considers publicly available information regarding the base salaries paid to similarly situated executive officers and other competitive market practices.

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        The salaries of the NEOs are reviewed on an annual basis, as well as at the time of promotion or any substantial change in responsibilities. The key factors in determining increases in salary level are relative performance and competitive pressures.

Annual Cash Bonuses

        Annual cash bonuses are intended to reward individual performance on an annual basis and can therefore be variable from year to year. Cash bonus awards for the NEOs are determined by the Compensation Committee on a discretionary basis based on performance criteria, particularly the Company's dividend performance as well as corporate and individual performance goals and other measures established by the Committee with the Chief Executive Officer's input (except with respect to his own performance criteria). Should actual performance exceed expected performance criteria, the Committee may adjust individual cash bonuses to take such superior performance into account.

Long-Term Incentive Awards

        Main Street's Board of Directors and stockholders approved the 2015 Equity and Incentive Plan in May 2015 to provide stock-based awards as long-term incentive compensation to employees, including the NEOs. The Company uses stock-based awards to (i) attract and retain key employees, (ii) motivate employees by means of performance-related incentives to achieve long-range performance goals, (iii) enable employees to participate in the Company's long-term growth in value and (iv) link employees' compensation to the long-term interests of stockholders. At the time of each award, the Compensation Committee will determine the terms of the award, including any performance period (or periods) and any performance objectives relating to vesting of the award. Prior to the adoption of the 2015 Equity and Incentive Plan, stock-based awards to employees were made under and are governed by the 2008 Equity Incentive Plan. Terms of the 2008 Equity Incentive Plan are substantially similar to the 2015 Equity and Incentive Plan. After adoption of the 2015 Equity and Incentive Plan no further awards have been, or will be, granted under the 2008 Equity Incentive Plan.

        Restricted Stock.    Main Street has received exemptive relief from the SEC that permits the Company to grant restricted stock in exchange for or in recognition of services by its executive officers and employees. Pursuant to the 2015 Equity and Incentive Plan, the Compensation Committee may award shares of restricted stock to plan participants in such amounts and on such terms as the Committee determines in its sole discretion, provided that such awards are consistent with the conditions set forth in the SEC's exemptive order. Each restricted stock grant will be for a fixed number of shares as set forth in an award agreement between the grantee and Main Street. Award agreements will set forth time and/or performance vesting schedules and other appropriate terms and/or restrictions with respect to awards, including rights to dividends and voting rights. Beginning in 2015, the Committee awarded restricted stock awards to employees, including NEOs, which vest in equal increments over a three-year time frame based on continued service during the vesting period. The Committee's previous practice had been to award restricted stock to employees which vested over a four-year time frame. The change to the vesting period was made to be more closely aligned with comparable companies.

        Options.    The Compensation Committee may also grant stock options to purchase Main Street's common stock (including incentive stock options and nonqualified stock options). The Committee expects that any options granted will represent a fixed number of shares of common stock, will have an exercise price equal to the fair market value of common stock on the date of grant, and will be exercisable, or "vested," at some later time after grant. Certain stock options may provide for vesting based on the grantee remaining employed by Main Street for a time certain and/or the grantee and/or the Company attaining specified performance criteria. To date, the Committee has not granted stock options to any NEO.

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        For a discussion of our policies related to employee and director hedging of economic risk of owning our securities, please see "Control Persons and Principal Stockholders".

Other Benefits

        Main Street's NEOs generally participate in the same benefit plans and programs as the Company's other employees, including comprehensive medical, dental and vision insurance, short term and long-term disability insurance and life insurance.

        Main Street maintains a 401(k) plan for all full-time employees who are at least 21 years of age through which the Company makes non-discretionary matching contributions to each participant's plan account on the participant's behalf. For each participating employee, the Company's contribution is a 100% match of the employee's contributions up to a 3% contribution level and a 50% match of the employee's contributions from a 3% to a 6% contribution level, with a maximum annual regular matching contribution of $12,375 during 2018. All contributions to the plan, including those made by the Company, vest immediately. The Board of Directors may also, at its sole discretion, provide that the Company will make additional contributions to employee 401(k) plan accounts, which would also vest immediately.

        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan to allow non-employee directors and certain key employees, including each of the NEOs, to defer receipt of some or all of their cash compensation, subject to certain limitations. Although not currently anticipated and subject to prior Compensation Committee approval, discretionary employer contributions are also permitted to the 2015 Deferred Compensation Plan. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan and are also able to direct investments made on their behalf among investment alternatives permitted from time to time under the plan, including phantom Main Street stock units for which shares of Main Street common stock will be issued upon distribution. The 2015 Deferred Compensation Plan became effective on January 1, 2016.

        In November 2018, our Board of Directors approved a policy to allow certain individuals, including members of our board of directors, certain senior officers and NEOs, to personally use Company aircraft when such aircraft is not being used by the Company. All such personal usage, as well as any request to bring a family member or guest on a business trip, is subject to approval by the Company's CEO or Chairman of the Board and reimbursement of the Company by the individual in an amount equal to the aggregate incremental cost to the Company for such usage. The Company retains authority to determine what flights may be scheduled based on the business needs of the Company, and the business needs of the Company take priority in any scheduling conflicts.

Perquisites

        The Company provides no other material benefits, perquisites or retirement benefits to the NEOs.

Potential Payments Upon Change in Control or Termination of Employment

        Unless the terms of an award provide otherwise, in the event of a specified transaction involving a "change in control" (as defined in the 2015 Equity and Incentive Plan) in which there is an acquiring or surviving entity, the Board of Directors may provide for the assumption of some or all outstanding awards, or for the grant of substitute awards, by the acquirer or survivor. In the event no such assumption or substitution occurs, each stock-based award, subject to its terms, will become fully vested or exercisable prior to the change in control on a basis that gives the holder of the award a reasonable opportunity, as determined by the Board of Directors, to participate as a stockholder in the change in

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control following vesting or exercise. The award will terminate upon consummation of the change in control.

        Transactions involving a "change in control" under the 2015 Equity and Incentive Plan include the following, other than where Main Street's stockholders continue to have substantially the same proportionate ownership in an entity which owns substantially all of Main Street's assets immediately following such transaction:

•

a single person or entity or group of persons and/or entities, other than Main Street, any of its employee benefit plans, a company owned by Main Street's stockholders in substantially the same proportions as their ownership in Main Street or an underwriter temporarily holding securities pursuant to an offering by Main Street, becomes the beneficial owner of more than 30% of the combined voting power of Main Street's voting securities then outstanding;



•

a change in the membership of the Board of Directors such that the individuals who, as of the effective date of the 2015 Equity and Incentive Plan, constitute the Board of Directors (the "Continuing Directors"), and any new director whose election or nomination to the Board of Directors was approved by a vote of at least a majority of the Continuing Directors, cease to constitute at least a majority of the Board;



•

a merger, reorganization or business combination of Main Street or one of its subsidiaries with or into any other entity, other than where the holders of Main Street's voting securities outstanding immediately before such transaction would represent immediately thereafter more than a majority of the combined voting power of the voting securities of Main Street or the surviving entity or the parent of such surviving entity;



•

a sale or disposition of all or substantially all of Main Street's assets, other than where the holders of Main Street's voting securities outstanding immediately before such transaction hold securities immediately thereafter which represent more than a majority of the combined voting power of the voting securities of the acquirer or the parent of such acquirer of such assets; or



•

Main Street's stockholders approve a plan of complete liquidation or dissolution of Main Street.

        Our restricted stock awards also provide that upon a participant's death, disability, involuntary termination without cause or voluntary termination with good reason (each as defined in the award agreement), the unvested shares of restricted stock will fully vest. The number of shares and value of unvested restricted stock for each NEO as of December 31, 2018 that would have vested under the acceleration scenarios described above is shown under "Compensation of Executive Officers—Outstanding Equity Awards at Fiscal Year-End."

        In addition, NEOs who participate in the 2015 Deferred Compensation Plan could receive a distribution of their balances in that plan in connection with their death, disability or termination of employment, depending on their distribution elections under the plan. The aggregate balance in the 2015 Deferred Compensation Plan of each NEO as of December 31, 2018 is shown under "Compensation of Executive Officers—Nonqualified Deferred Compensation."

        Other than the accelerated vesting of restricted stock and amounts due under the 2015 Deferred Compensation Plan, we currently would not incur any other payment obligations to our NEOs in the event of a change in control or any of the aforementioned causes of termination of employment.

1940 Act Restrictions on Company Performance Based Compensation

        The 1940 Act provides that a BDC such as Main Street may maintain either an equity incentive plan or a "profit-sharing plan", but not both, for its NEOs and other employees. The Compensation Committee believes that equity incentives closely align the interests of NEOs and employees with those of the Company's stockholders. Accordingly, Main Street has adopted and maintained equity incentive

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plans for its NEOs and employees since 2008. As a result, the 1940 Act prohibits Main Street from having a "profit-sharing plan."

        The term "profit-sharing plan" is very broadly defined in the 1940 Act but in this context is generally viewed as referring to incentive and other compensation being directly tied to a company's gross or net income or any other indicia of the company's overall financial performance, such as realized gains or losses and unrealized appreciation or depreciation on investments. In this regard, the SEC has indicated that a compensation program possesses profit-sharing characteristics if a company is obligated to make payments under the program based on company performance metrics.

        Due to these restrictions imposed by the 1940 Act, the Compensation Committee is not permitted to use nondiscretionary or formulaic Company performance goals or criteria to determine executive incentive compensation. Instead, the Committee considers overall Company performance along with other factors, including individual performance criteria, and uses its discretion in determining the appropriate compensation for NEOs and other key employees. The Compensation Committee's objective is to work within the 1940 Act regulatory framework to establish appropriate compensation levels, maintain pay-for-performance alignment and implement compensation best practices.

Tax Deductibility of Compensation

        Section 162(m) of the Internal Revenue Code of 1986, as amended, generally disallows a deduction to public companies to the extent of excess annual compensation over $1 million paid to certain executive officers. Section 162(m) was amended and expanded by the Tax Cuts and Jobs Act enacted at the end of 2017. For amounts paid in 2018 and later years, the Section 162(m) deduction limit applies to an expanded group of current and former executive officers with limited exceptions. In addition, the exception for performance-based compensation is no longer available starting in 2018. Accordingly, annual cash bonuses paid for 2018 performance as well as equity awards granted in 2018 and later years are subject to the deduction limit of Section 162(m). Therefore, to the extent any of the covered executive officers are paid compensation in excess of $1 million for any year after 2017, Main Street generally cannot deduct such excess compensation for U.S. federal income tax purposes.

        While the Compensation Committee considers the deductibility of compensation as one factor in determining executive compensation, the Compensation Committee also considers other factors in making compensation decisions as noted herein and retains the flexibility to authorize amounts and forms of compensation that it determines to be consistent with the goals of our executive compensation program even if such compensation is not deductible by the Company for tax purposes.

Stockholder Advisory Vote on Executive Compensation

        At our 2018 annual meeting of stockholders, our stockholders provided an advisory vote with 91% of the votes cast approving our compensation philosophy, policies and procedures and the 2017 fiscal year compensation of our NEOs (the "Advisory Vote"). Subsequently, the Compensation Committee considered the results of the Advisory Vote in determining compensation policies and decisions of the Company. The Advisory Vote affected the Company's executive compensation decisions and policies by reaffirming the Company's compensation philosophies, and the Compensation Committee will continue to use these philosophies and past practice in determining future compensation decisions.

2018 Compensation Determination

        The Compensation Committee analyzed the competitiveness of the components of compensation described above on both an individual and aggregate basis. The Committee believes that the total compensation paid to the NEOs for the fiscal year ended December 31, 2018, is consistent with the overall objectives of Main Street's executive compensation program.

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Base Salary

        The Compensation Committee annually reviews the base salary of each executive officer, including each NEO, and determines whether or not to increase it in its sole discretion. Increases to base salary can be awarded to recognize, among other things, relative performance, relative cost of living and competitive pressures.

        In 2018, the Compensation Committee approved base salary increases for each NEO in recognition each NEO's and the Company's performance for the year and also to more closely align each NEO's compensation with similar executive officers of comparable companies.

        The amount of annual base salary paid to each NEO for 2018 is presented under the caption entitled "Compensation of Executive Officers—Summary Compensation Table." The Committee believes that the salary changes and resulting base salaries were competitive in the market place and appropriate for Main Street executives as a key component of an overall compensation package.

Annual Cash Incentive Bonus

        Cash bonuses are determined annually by the Compensation Committee and are based on individual and corporate performance objectives coupled with Committee discretion as appropriate. The 2018 performance criteria used for determining the cash bonuses for NEOs included, among other things, the following:

•

Achievement of corporate objectives, particularly those related to the maintenance and growth of dividends and preservation of capital through maintenance and growth of net asset value per share;



•

Individual performance and achievement of individual goals, as well as the contribution to corporate objectives;



•

Maintaining liquidity and capital flexibility to accomplish the Company's business objectives;



•

Maintaining the highest ethical standards, internal controls and adherence to regulatory requirements; and



•

Appropriate and planned development of personnel.

        The Company paid cash bonuses to NEOs for 2018 performance in recognition of Main Street's strong financial results. The NEOs contributed significantly to the Company's performance. Major achievements considered by the Compensation Committee included increased distributable net investment income per share, increased regular monthly dividends per share, increased net asset value per share, growth of the investment portfolio, continuation of the Company's low total operating cost structure in comparison to peer organizations, maintaining an investment grade rating from Standard & Poor's Ratings Services, use of our at-the-market equity offering to issue equity efficiently as needed while maintaining a conservative leverage ratio, low employee turnover and development of talented personnel. The Compensation Committee did not weight these achievements and used discretion in determining the cash bonus amount allocated to each executive. In summary, the performance of the NEO group and the management team overall was at a consistent high level in 2018 resulting in excellent financial results. As approved by the Compensation Committee, Mr. Foster's cash bonus related to 2018 performance was issued in the form of shares of restricted stock under the 2015 Equity and Incentive Plan instead of cash, as discussed further in "Compensation of Executive Officers—Summary Compensation Table".

        The amount of cash bonus paid to each NEO for 2018 is presented under the caption entitled "Compensation of Executive Officers—Summary Compensation Table." The Committee believes that

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these cash bonus awards are individually appropriate based on 2018 performance. Such bonuses comprise a key component of the Company's overall compensation program.

Long-Term Incentive Awards

        The Company granted restricted shares to our NEOs in 2018 to recognize individual contributions to corporate strategic priorities and to the long-term performance of the Company. Other objectives of restricted stock awards were to assist with retention, align NEO interests with stockholders' and to provide competitive total direct compensation. Contributions to the future success of the Company include expanded roles of NEOs within the Company, recruitment and development of personnel, advancement of strategic initiatives with benefits beyond the current year, development of appropriate capital structure alternatives and enhancement of the Company's reputation with key constituents.

        The amount of restricted shares granted to each NEO in 2018 is presented under the caption entitled "Compensation of Executive Officers—Grants of Plan-Based Awards." The Committee is currently assessing the potential for long-term incentive compensation through grants of restricted shares to our NEOs for 2019, which are expected to be awarded in April 2019.

Risk Management and Compensation Policies and Practices

        We believe that risks arising from our compensation policies and practices for our employees are not reasonably likely to have a material adverse effect on the Company. In addition, the Compensation Committee believes that the mix and design of the elements of executive compensation do not encourage management to assume excessive risks.

        The Compensation Committee has reviewed the elements of executive compensation to determine whether any portion of executive compensation encourages excessive risk taking and concluded:

•

compensation is allocated among base salary and short and long-term compensation opportunities in such a way as to not encourage excessive risk-taking;



•

significant weighting towards long-term incentive compensation discourages short-term risk taking;



•

executive goals are appropriately established across several key metrics and criteria in order to avoid an outcome where the failure to achieve any individual target would result in a large percentage loss of compensation; and



•

multi-year vesting of restricted stock coupled with share ownership guidelines properly account for the time horizon of risks.

        Finally, in addition to the factors described above, incentive compensation decisions include subjective considerations that restrain the influence of formulae or objective driven determinations that might lead to excessive risk taking.

COMPENSATION OF EXECUTIVE OFFICERS

        The following table summarizes the compensation of our Named Executive Officers, or NEOs, for the fiscal year ended December 31, 2018.

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Summary Compensation Table

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Grants of Plan-Based Awards

        The following table sets forth information regarding restricted stock awards granted to our NEOs in fiscal year 2018:

Outstanding Equity Awards at Fiscal Year-End

        The following table sets forth the awards of restricted stock for which forfeiture provisions have not lapsed and remain outstanding at December 31, 2018:

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Equity Awards Vested in Fiscal Year

        The following table sets forth information regarding shares of restricted stock for which forfeiture restrictions lapsed during the fiscal year ended December 31, 2018:

Nonqualified Deferred Compensation

        The following table sets forth information regarding the activity during the fiscal year ended December 31, 2018 related to the accounts of our NEOs under the 2015 Deferred Compensation Plan:

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        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan, an unfunded, nonqualified deferred compensation plan, to allow non-employee directors and certain key employees, including each of the NEOs, to defer receipt of some or all of their cash compensation, subject to certain limitations. Pursuant to the 2015 Deferred Compensation Plan, executives may contribute on a pre-tax basis up to 100% of their salary, bonus and dividends paid on shares of unvested Company stock. Although not currently anticipated and subject to prior Compensation Committee approval, discretionary employer contributions are also permitted to the 2015 Deferred Compensation Plan. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan. Amounts deferred under the plan earn a return based on the returns on certain investment alternatives permitted under the plan, including phantom Main Street stock units, as designated by the participant. The 2015 Deferred Compensation Plan became effective on January 1, 2016.

Potential Payments Upon Change in Control or Termination of Employment

        As described in "Compensation Discussion and Analysis," our restricted stock awards to employees, including NEOs, provide that upon certain transactions involving a change in control, or upon a participant's death, disability, involuntary termination without cause or voluntary termination with good reason (each as defined in the award agreement), the unvested shares of restricted stock will fully vest. The number of shares and value of unvested restricted stock for each NEO as of December 31, 2018 that would have vested under the acceleration scenarios described above is shown under the heading "—Outstanding Equity Awards at Fiscal Year-End."

        In addition, NEOs who participate in the 2015 Deferred Compensation Plan could receive a distribution of their balances in that plan in connection with their death, disability or termination of employment, depending on their distribution elections under the plan. The aggregate balance in the 2015 Deferred Compensation Plan of each NEO as of December 31, 2018 is shown under the heading "—Nonqualified Deferred Compensation."

        Other than the accelerated vesting of restricted stock and amounts due under the 2015 Deferred Compensation Plan, we currently would not incur any other payment obligations to our NEOs in the event of a change in control or any of the aforementioned causes of termination of employment.

Chief Executive Officer Pay Ratio

        For 2018, our last completed fiscal year, the median of the annual total compensation of all of our employees (other than Mr. Hyzak, our Chief Executive Officer (our "CEO")) was $154,426, and the annualized total compensation of our CEO was $3,952,209. Based on this information, our CEO's 2018 annual total compensation was approximately 26 times that of the median of the 2018 annual total compensation of all of our employees (the "CEO Pay Ratio").

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        We selected December 31, 2018 as the date used to identify our "median employee" whose annual total compensation was the median of the annual total compensation of all our employees (other than our CEO) for 2018. As of December 31, 2018, our employee population consisted of 66 individuals, all located in our Houston, Texas office. We compared the annual total compensation for our employee population in accordance with the requirements of Item 402(c)(2)(x) of Regulation S-K, which included salary, bonus, stock awards and employer matching contributions to employee accounts in our 401(k) plan. In making this determination, we annualized the compensation of 17 employees who were hired in 2018 but did not work for us the entire fiscal year.

        As discussed elsewhere in this prospectus supplement, the Company had two individuals serving in the role of CEO during 2018. We elected to use the compensation of Mr. Hyzak, the active CEO as of December 31, 2018, for purposes of determining the CEO Pay Ratio. Mr. Hyzak became CEO in November 2018. In determining Mr. Hyzak's compensation for purposes of the CEO Pay Ratio, we adjusted the compensation reported in the Summary Compensation Table to reflect his compensation as if he were CEO for the full calendar year. For purposes of calculating the CEO Pay Ratio, this resulted in total annual compensation of $3,952,209 for the CEO as opposed to the amount shown on the Summary Compensation Table of $3,242,451.

CERTAIN RELATIONSHIPS AND RELATED PARTY TRANSACTIONS

        We have procedures in place for the review, approval and monitoring of transactions involving us and certain persons related to us. As a BDC, the 1940 Act restricts us from participating in transactions with any persons affiliated with us, including our officers, directors and employees and any person controlling or under common control with us, subject to certain exceptions.

        In the ordinary course of business, we enter into transactions with portfolio companies that may be considered related party transactions. We have implemented certain policies and procedures, both written and unwritten, to ensure that we do not engage in any prohibited transactions with any persons affiliated with us. If such affiliations are found to exist, we seek Board and/or appropriate Board committee review and approval or exemptive relief for such transactions, as appropriate.

        In addition, our Code of Business Conduct and Ethics, which is applicable to all of our employees, officers and directors, requires that all employees, officers and directors avoid any conflict, or the appearance of a conflict, between an individual's personal interests and our interests. Our Code of Business Conduct and Ethics is available at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

CONTROL PERSONS AND PRINCIPAL STOCKHOLDERS

        The following table sets forth, as of March 6, 2019, information with respect to the beneficial ownership of our common stock by:

•

each person known to us to beneficially own more than five percent of the outstanding shares of our common stock;



•

each of our directors and executive officers; and



•

all of our directors and executive officers as a group.

        Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to the securities. There is no common stock subject to options that are currently exercisable or exercisable within 60 days of March 6, 2019. Percentage of beneficial ownership is based on 61,888,278 shares of common stock outstanding as of March 6, 2019.

        Unless otherwise indicated, to our knowledge, each stockholder listed below has sole voting and investment power with respect to the shares beneficially owned by the stockholder, and maintains an

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address c/o Main Street Capital Corporation. Our address is 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056.

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        The Board of Directors has established stock ownership guidelines pursuant to which independent directors and certain key employees, including each executive officer listed in the table above, are required to achieve and maintain minimum levels of stock ownership. Our Corporate Governance and Stock Ownership Guidelines may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

        Our insider trading policy prohibits all directors, officers and employees from, directly or indirectly, trading while in the possession of material nonpublic information related to the Company and from engaging in short sales and short-term or other speculative trading of our securities and any transactions that hedge or offset, or are designed to hedge or offset, any decrease in the market value of securities issued by us. Prohibited hedging activity includes market transactions in puts, calls and other derivatives and the purchase of prepaid variable forward contracts, equity swaps, collars and exchange funds related to securities issued by us. Pledging our securities in a margin account or as collateral for a loan is also prohibited under the policy except in limited circumstances that are pre-approved by our chief compliance officer.

        The following table sets forth, as of March 6, 2019, the dollar range of our equity securities that is beneficially owned by each of our directors.

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DESCRIPTION OF THE NOTES

        The following description of the particular terms of the        % Notes due                supplements and, to the extent inconsistent with, replaces the description of the general terms and provisions of the debt securities set forth in the accompanying prospectus.

        We will issue the Notes under a base indenture dated as of April 2, 2013, between us and The Bank of New York Mellon Trust Company, N.A., as trustee, or "the trustee," as supplemented by a separate supplemental indenture, to be dated as of the settlement date for the Notes. As used in this section, all references to the indenture mean the base indenture as supplemented by the supplemental indenture. The terms of the Notes include those expressly set forth in the indenture and those made part of the indenture by reference to the Trust Indenture Act of 1939, as amended.

        The following description is a summary of the material provisions of the Notes and the indenture and does not purport to be complete. This summary is subject to and is qualified by reference to all the provisions of the Notes and the indenture, including the definitions of certain terms used in the indenture. We urge you to read these documents because they, and not this description, define your rights as a holder of the Notes. You may request a copy of the indenture from us by making a written request to Main Street Capital Corporation, 1300 Post Oak Boulevard, 8th Floor, Houston, TX 77056, or by calling us collect at (713) 350-6000, or by visiting our website at www.mainstcapital.com. The information on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus.

        For purposes of this description, references to "we," "our" and "us" refer only to Main Street Capital Corporation and not to any of our current or future subsidiaries and references to "subsidiaries" refer only to our consolidated subsidiaries and exclude any investments held by Main Street Capital Corporation in the ordinary course of business which are not, under GAAP, consolidated on the financial statements of Main Street Capital Corporation and its subsidiaries.

General

        The Notes:

•

will be our general unsecured, senior obligations;



•

will initially be issued in an aggregate principal amount of $            ;



•

will mature on                ,                 , unless earlier redeemed or repurchased, as discussed below;



•

will bear cash interest from April     , 2019 at an annual rate of        % payable semiannually on                and                 of each year, beginning on                ,                 ;



•

will be subject to redemption at our option as described under "—Optional Redemption;"



•

will be subject to repurchase by us at the option of the holders following a Change of Control Repurchase Event (as defined below under "—Offer to Repurchase Upon a Change of Control Repurchase Event"), at a repurchase price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the date of repurchase;



•

will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof; and



•

will be represented by one or more registered Notes in global form, but in certain limited circumstances may be represented by Notes in definitive form. See "—Book-Entry, Settlement and Clearance."

        The indenture does not limit the amount of debt that may be issued by us or our subsidiaries under the indenture or otherwise, but does contain a covenant regarding our asset coverage that would

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have to be satisfied at the time of our incurrence of additional indebtedness. See "—Covenants—Other Covenants." The indenture does not contain any financial covenants and does not restrict us from paying dividends or distributions or issuing or repurchasing our other securities. Other than restrictions described under "—Offer to Repurchase Upon a Change of Control Repurchase Event" and "—Merger, Consolidation or Sale of Assets" below, the indenture does not contain any covenants or other provisions designed to afford holders of the Notes protection in the event of a highly leveraged transaction involving us or in the event of a decline in our credit rating as the result of a takeover, recapitalization, highly leveraged transaction or similar restructuring involving us that could adversely affect such holders.

        We may, without the consent of the holders, issue additional Notes under the indenture with the same terms as the Notes offered hereby in an unlimited aggregate principal amount; provided that, if such additional Notes are not fungible with the Notes offered hereby (or any other tranche of additional Notes) for U.S. federal income tax purposes, then such additional Notes will have different CUSIP numbers from the Notes offered hereby (and any such other tranche of additional Notes).

        We do not intend to list the Notes on any securities exchange or any automated dealer quotation system.

Payments on the Notes; Paying Agent and Registrar; Transfer and Exchange

        We will pay the principal of, and interest on, the Notes in global form registered in the name of or held by DTC, or its nominee in immediately available funds to DTC or its nominee, as the case may be, as the registered holder of such Global Note (as defined below).

        Payment of principal of (and premium, if any) and any such interest on the Notes will be made at the corporate trust office of the trustee in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that at our option payment of interest may be made by check mailed to the address of the person entitled thereto as such address shall appear in the security register.

        A holder of the Notes may transfer or exchange Notes at the office of the security registrar in accordance with the indenture. The security registrar and the trustee may require a holder, among other things, to furnish appropriate endorsements and transfer documents. No service charge will be imposed by us, the trustee or the security registrar for any registration of transfer or exchange of Notes, but we may require a holder to pay a sum sufficient to cover any transfer tax or other similar governmental charge required by law or permitted by the indenture.

        The registered holder of a Note will be treated as its owner for all purposes.

Interest

        The Notes will bear cash interest at a rate of        % per year until maturity. Interest on the Notes will accrue from April     , 2019 or from the most recent date on which interest has been paid or duly provided for. Interest will be payable semiannually in arrears on                and                 of each year, beginning on                ,                 .

        Interest will be paid to the person in whose name a Note is registered at 5:00 p.m. New York City time, or the close of business, on                or                , as the case may be, immediately preceding the relevant interest payment date, or each, a "regular record date." Interest on the Notes will be computed on the basis of a 360-day year composed of twelve 30-day months.

        If any interest payment date, the maturity date or any earlier required repurchase date upon a Change of Control Repurchase Event (defined below) of a Note falls on a day that is not a business day, the required payment will be made on the next succeeding business day and no interest on such

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payment will accrue in respect of the delay. The term "business day" means, with respect to any Note, each Monday, Tuesday, Wednesday, Thursday, and Friday that is not a day on which banking institutions in The City of New York are authorized or obligated by law or executive order to close.

Ranking

        The Notes will be our general unsecured obligations that rank pari passu with our existing and future general unsecured and senior unsecured indebtedness, including our 4.50% Notes due 2019 and our 4.50% Notes due 2022. The Notes will rank senior to any of our future indebtedness that expressly states it is subordinated to the Notes and effectively subordinated to all of our existing and future secured indebtedness (including indebtedness that is initially unsecured, but to which we subsequently grant security), to the extent of the value of the assets securing such indebtedness, including approximately $293.0 million of borrowings outstanding as of April 12, 2019 under our Credit Facility to the extent of the value of the assets securing the Credit Facility. The Notes will rank structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, financing vehicles or similar facilities, including the Funds' $321.8 million of SBIC debentures outstanding as of April 12, 2019. In the event of our bankruptcy, liquidation, reorganization or other winding up, our assets that secure secured debt will be available to pay obligations on the Notes only after all indebtedness under such secured debt has been repaid in full from such assets. We advise you that there may not be sufficient assets remaining to pay amounts due on any or all the Notes then outstanding.

Optional Redemption

        We may redeem some or all of the Notes at any time, or from time to time. If we choose to redeem any Notes prior to maturity, we will pay a redemption price (as determined by us) equal to the greater of the following amounts, plus, in each case, accrued and unpaid interest to, but excluding, the redemption date:

•

100% of the principal amount of the Notes to be redeemed, or



•

the sum of the present values of the remaining scheduled payments of principal and interest (exclusive of accrued and unpaid interest to the date of redemption) on the Notes to be redeemed, discounted to the redemption date on a semiannual basis (assuming a 360-day year consisting of twelve 30-day months) using the applicable Treasury Rate plus                basis points.

        If we choose to redeem any Notes, we will deliver a notice of redemption to holders of the Notes not less than 30 nor more than 60 days before the redemption date. If we are redeeming less than all of the Notes, the particular Notes to be redeemed will be selected in accordance with the indenture and, so long as the Notes are registered to DTC or its nominee, in accordance with the procedures of DTC; provided, however, that no such partial redemption shall reduce the portion of the principal amount of a Note not redeemed to less than $2,000. Unless we default in payment of the redemption price, on and after the redemption date, interest will cease to accrue on the Notes or portions of the Notes called for redemption.

        For purposes of calculating the redemption price in connection with the redemption of the Notes, on any redemption date, the following terms have the meanings set forth below:

        "Treasury Rate" means, with respect to any redemption date, the rate per annum equal to the semiannual equivalent yield-to-maturity of the Comparable Treasury Issue (computed as of the third business day immediately preceding the redemption), assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date. The redemption price and the Treasury Rate will be determined by us.

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        "Comparable Treasury Issue" means the United States Treasury security selected by the Reference Treasury Dealer as having a maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financing practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of the Notes being redeemed.

        "Comparable Treasury Price" means (1) the average of the remaining Reference Treasury Dealer Quotations for the redemption date, after excluding the highest and lowest Reference Treasury Dealer Quotations, or (2) if the Quotation Agent obtains fewer than four such reference treasury dealer quotations, the average of all such quotations.

        "Quotation Agent" means a Reference Treasury Dealer selected by us.

        "Reference Treasury Dealer" means RBC Capital Markets, LLC and four other financial institutions selected by us, or their affiliates, which are primary U.S. government securities dealers and their respective successors; provided, however, that if any of the foregoing or their affiliates shall cease to be a primary U.S. government securities dealer in the United States, or a "Primary Treasury Dealer," we shall select another Primary Treasury Dealer.

        "Reference Treasury Dealer Quotations" means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Quotation Agent, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Quotation Agent by such Reference Treasury Dealer at 3:30 p.m. New York City time on the third business day preceding such redemption date.

        All determinations made by any Reference Treasury Dealer, including the Quotation Agent, with respect to determining the redemption price will be final and binding absent manifest error.

        In addition, we may at any time purchase any of the Notes in the open market or in private transactions, at differing prices, or by tender, subject to applicable law.

Offer to Repurchase Upon a Change of Control Repurchase Event

        If a Change of Control Repurchase Event occurs, unless we have exercised our right to redeem the Notes in full, we will make an offer to each holder of Notes to repurchase all or any part (in minimum denominations of $2,000 and integral multiples of $1,000 principal amount) of that holder's Notes at a repurchase price in cash equal to 100% of the aggregate principal amount of Notes repurchased plus any accrued and unpaid interest on the Notes repurchased to, but excluding, the date of repurchase. Within 30 days following any Change of Control Repurchase Event or, at our option, prior to any Change of Control, but after the public announcement of the Change of Control, we will mail a notice to each holder describing the transaction or transactions that constitute or may constitute the Change of Control Repurchase Event and offering to repurchase Notes on the payment date specified in the notice, which date will be no earlier than 30 days and no later than 60 days from the date such notice is mailed. The notice shall, if mailed prior to the date of consummation of the Change of Control, state that the offer to purchase is conditioned on the Change of Control Repurchase Event occurring on or prior to the payment date specified in the notice. We will comply with the requirements of Rule 14e-1 promulgated under the Exchange Act and any other securities laws and regulations thereunder to the extent those laws and regulations are applicable in connection with the repurchase of the Notes as a result of a Change of Control Repurchase Event. To the extent that the provisions of any securities laws or regulations conflict with the Change of Control Repurchase Event provisions of the Notes, we will comply with the applicable securities laws and regulations and will not be deemed to have breached our obligations under the Change of Control Repurchase Event provisions of the Notes by virtue of such conflict.

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        On the Change of Control Repurchase Event payment date, subject to extension if necessary to comply with the provisions of the 1940 Act and the rules and regulations promulgated thereunder, we will, to the extent lawful:

•

accept for payment all Notes or portions of Notes properly tendered pursuant to our offer;



•

deposit with the paying agent an amount equal to the aggregate purchase price in respect of all Notes or portions of Notes properly tendered; and



•