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INDEX TO FINANCIAL STATEMENTS

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Filed Pursuant to Rule 497
Registration Statement No. 333-203147

PROSPECTUS SUPPLEMENT
(to Prospectus dated May 16, 2016)

Up to 2,500,000 Shares
Common Stock

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         We have entered into separate equity distribution agreements, each dated November 18, 2016, with Goldman, Sachs & Co., Raymond James & Associates, Inc., RBC Capital Markets, LLC and Robert W. Baird & Co. Incorporated, each a "Sales Agent" and, collectively, the "Sales Agents," relating to the shares of common stock offered by this prospectus supplement and the accompanying prospectus. The equity distribution agreements provide that we may offer and sell up to 2,500,000 shares of our common stock from time to time through the Sales Agents. Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be "at the market," as defined in Rule 415 under the Securities Act of 1933, as amended, including sales made directly on the New York Stock Exchange ("NYSE") or similar securities exchange or sales made to or through a market maker other than on an exchange, at prices related to the prevailing market prices or at negotiated prices. See "Plan of Distribution." As of the date of this prospectus supplement, we have not sold any shares of our common stock under the equity distribution agreements.

         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 $15 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.

         Our common stock is listed on the NYSE under the symbol "MAIN." On November 17, 2016, the last reported sale price of our common stock on the NYSE was $36.31 per share, and the net asset value per share of our common stock on September 30, 2016 (the last date prior to the date of this prospectus supplement on which we determined our net asset value per share) was $21.62.

         Under the terms of the equity distribution agreements, the Sales Agents will receive a commission from us equal to up to 1.0% of the gross sales price of any shares of our common stock sold through the Sales Agents under the equity distribution agreements. The Sales Agents are not required to sell any specific number or dollar amount of common stock, but will use their commercially reasonable efforts consistent with their sales and trading practices to sell the shares of our common stock offered by this prospectus supplement and the accompanying prospectus. We may also sell shares of our common stock to a Sales Agent, as principal for its own respective account, at a price agreed upon at the time of sale. If we sell shares to a Sales Agent as principal, we will enter into a separate terms agreement with the applicable Sales Agent, setting forth the terms of such transaction, and we will describe the agreement in a separate prospectus supplement. See "Plan of Distribution" beginning on page S-37 of this prospectus supplement.

         Investing in our common stock involves a high degree of risk, and should be considered highly speculative. See "Risk Factors" beginning on page 14 of the accompanying prospectus to read about factors you should consider, including the risk of leverage and dilution, before investing in our common stock.

         This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in our common stock. 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.

The date of this prospectus supplement is November 18, 2016

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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 common stock 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 common stock 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 Sales Agents 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 shares of our common stock 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 common stock. 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 section titled "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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USE OF PROCEEDS

        Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be "at the market" as defined in Rule 415 under the Securities Act, including sales made directly on the NYSE or sales made to or through a market maker other than on an exchange. There is no guarantee that there will be any sales of our common stock pursuant to this prospectus supplement and the accompanying prospectus. Actual sales, if any, of our common stock under this prospectus supplement and the accompanying prospectus may be less than as set forth in this paragraph depending on, among other things, the market price of our common stock at the time of any such sale. As a result, the actual net proceeds we receive may be more or less than the amount of net proceeds estimated in this prospectus supplement. Assuming the sale of all 2,500,000 shares of common stock available for sale under the equity distribution agreements with the Sales Agents at a price of $36.31 per share (the last reported sales price of our common stock on November 17, 2016), we estimate that the net proceeds of this offering will be approximately $89.6 million after deducting the estimated sales commission payable to the Sales Agent and our estimated offering expenses.

        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 November 17, 2016, we had approximately $320.0 million outstanding under our Credit Facility. Our Credit Facility matures in September 2021, unless extended, and bears interest, at our election, on a per annum basis equal to (A)(i) the applicable LIBOR rate plus 1.875% or (ii) the applicable base rate plus 0.875% so long as we maintain an investment grade rating and satisfy certain agreed upon excess collateral and leverage requirements, (B) 0.125% higher in each case so long as we maintain an investment grade rating but not the agreed upon excess collateral and/or leverage requirements, and (C) 0.375% higher in each case so long as we do not maintain an investment grade rating. Amounts repaid under our Credit Facility will remain available for future borrowings.

        Affiliates of Goldman, Sachs & Co., Raymond James & Associates, Inc. and RBC Capital Markets, LLC, Sales Agents 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 this Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Plan of Distribution" below.

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SELECTED FINANCIAL DATA

        The selected financial and other data below reflects the consolidated financial condition and the consolidated statement of operations of Main Street and its subsidiaries as of and for the years ended December 31, 2015, 2014, 2013, 2012 and 2011, and as of September 30, 2016 and for the nine months ended September 30, 2016 and 2015. The selected financial data as of and for the years ended December 31, 2015, 2014, 2013, 2012 and 2011 have been derived from consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. The selected financial data as of September 30, 2016, and for the nine months ended September 30, 2016 and 2015, have been derived from unaudited financial data but, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary to present fairly the financial condition and operating results for such interim periods. Interim results as of and for the nine months ended September 30, 2016 are not necessarily indicative of the results that may be expected for the year ending December 31, 2016. 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 financial statements and related notes thereto in the accompanying prospectus and "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in this prospectus supplement.

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

        The following discussion should be read in conjunction with our interim financial statements and 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 "Forward-Looking Statements" 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 diverse 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 receive 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 ("RIA") under Investment Advisers Act of 1940, as amended (the "Advisers Act"). 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 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. The External Investment Manager is also a direct wholly owned subsidiary that has elected to be a taxable entity. The Taxable Subsidiaries and the External Investment Manager are each taxed at their normal corporate tax rates based on their taxable income.

        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 $15 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 more 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,

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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.

        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 September 30, 2016 and December 31, 2015 (this information excludes the Other Portfolio investments and the External Investment Manager which are discussed further below):

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        As of September 30, 2016, we had Other Portfolio investments in ten companies, collectively totaling approximately $94.8 million in fair value and approximately $101.3 million in cost basis and which comprised 4.9% of our Investment Portfolio (as defined in "—Critical Accounting Policies—Basis of Presentation" below) at fair value. As of December 31, 2015, we had Other Portfolio investments in ten companies, collectively totaling approximately $74.8 million in fair value and approximately $75.2 million in cost basis and which comprised approximately 4.2% 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 September 30, 2016, there was no cost basis in this investment and the investment had a fair value of $30.1 million, which comprised 1.6% of our Investment Portfolio at fair value. As of December 31, 2015, there was no cost basis in this investment and the investment had a fair value of $27.3 million, which comprised 1.5% 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 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.

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        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 three months ended September 30, 2016, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.5% on an annualized basis, compared to 1.3% on an annualized basis for the three months ended September 30, 2015. For the nine months ended September 30, 2016, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.4% on an annualized basis, which is consistent with the ratio on an annualized basis for the nine months ended September 30, 2015 and for the year ended December 31, 2015.

        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-publicly traded 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. Based upon several fee waiver agreements with HMS Income and HMS Adviser, the External Investment Manager did not begin accruing the base management fee and incentive fees, if any, until January 1, 2014. The External Investment Manager has conditionally agreed to waive a limited amount of the incentive fees otherwise earned. During the three months ended September 30, 2016 and 2015, the External Investment Manager earned $2.5 million and $2.1 million, respectively, of management fees (net of fees waived, if any) under the sub-advisory agreement with HMS Adviser. During the nine months ended September 30, 2016 and 2015, the External Investment Manager earned $7.1 million and $5.5 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

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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.

CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our 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, but excludes all "Marketable securities and idle funds investments". "Marketable securities and idle funds investments" are classified as financial instruments and are reported separately on our consolidated balance sheets and consolidated schedules of investments due to the nature of such investments. Our results of operations for the three and nine months ended September 30, 2016 and 2015, cash flows for the nine months ended September 30, 2016 and 2015, and financial position as of September 30, 2016 and December 31, 2015, are presented on a consolidated basis. The effects of all intercompany transactions between us and our consolidated subsidiaries have been eliminated in consolidation.

        Our accompanying unaudited consolidated financial statements are presented in conformity with U.S. GAAP for interim financial information and pursuant to the requirements for reporting on Form 10-Q and Article 10 of Regulation S-X. Accordingly, certain disclosures accompanying annual financial statements prepared in accordance with U.S. GAAP are omitted. In the opinion of management, the unaudited consolidated financial results included herein contain all adjustments, consisting solely of normal recurring accruals, considered necessary for the fair presentation of financial statements for the interim periods included herein. The results of operations for the three and nine months ended September 30, 2016 and 2015 are not necessarily indicative of the operating results to be expected for the full year. Also, the unaudited financial statements and notes should be read in conjunction with the audited financial statements and notes thereto for the year ended December 31, 2015. Financial statements prepared on a U.S. GAAP basis require management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed herein.

        Under regulations pursuant to Article 6 of Regulation S-X applicable to BDCs and Accounting Standards Codification ("Codification" or "ASC") 946, Financial Services—Investment Companies ("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. Our consolidated financial statements also include the financial position and operating results for our wholly owned operating subsidiary, Main Street Capital Partners, LLC, ("MSCP"), as the wholly owned subsidiary provides all of its services directly or indirectly to Main Street or our portfolio companies. 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 Change

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in 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 September 30, 2016 and December 31, 2015, 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 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 September 30, 2016 and December 31, 2015 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 fully impaired, sold or written off, we remove it from non-accrual status.

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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 interest 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 three months ended September 30, 2016 and 2015, (i) approximately 4.0% and 2.2%, respectively, of our total investment income was attributable to PIK interest income not paid currently in cash and (ii) approximately 1.8% and 1.2%, respectively, of our total investment income was attributable to cumulative dividend income not paid currently in cash. For the nine months ended September 30, 2016 and 2015, (i) approximately 3.7% and 2.1%, respectively, of our total investment income was attributable to PIK interest income not paid currently in cash and (ii) approximately 1.1% and 1.0%, respectively, of our total investment income was attributable to cumulative dividend income not paid currently in cash.

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.

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 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 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

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and to continue to comply with the "source-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. This income tax expense, or benefit, if any, and the related tax assets and liabilities, are reflected in our consolidated financial statements.

        MSCP is included in our consolidated financial statements for financial reporting purposes. For tax purposes, MSCP has elected to be treated as a taxable entity, and therefore is not consolidated with MSCC for income tax purposes and is taxed at normal corporate tax rates based on its taxable income and, as a result of its activities, may generate income tax expense or benefit. The taxable income, or loss, of MSCP may differ from its book income, or loss, due to temporary book and tax timing differences and permanent differences. This income tax expense, or benefit, if any, and the related tax assets and liabilities, are reflected in our consolidated financial statements.

        The Taxable Subsidiaries and MSCP 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 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 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, generally bear interest at fixed rates, and generally have a term of between five and seven years from the original investment date. In most LMM portfolio companies, 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 $15 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

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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.

        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. In the first quarter of 2014, we began allocating costs to the External Investment Manager pursuant to the sharing agreement. Our total expenses for the three months ended September 30, 2016 and 2015 are net of expenses allocated to the External Investment Manager of $1.2 million and $1.1 million, respectively. Our total expenses for the nine months ended September 30, 2016 and 2015 are net of expenses allocated to the External Investment Manager of $3.7 million and $3.1 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 dividend income from the External Investment Manager. For the three months ended September 30, 2016 and 2015, the total contribution to our net investment income was $2.0 million and $1.8 million, respectively. For the nine months ended September 30, 2016 and 2015, the total contribution to our net investment income was $5.8 million and $4.7 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 September 30, 2016 and December 31, 2015 (this information excludes the Other Portfolio investments and the External Investment Manager).

 

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        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" contained in the accompanying prospectus for a more complete discussion of the risks involved with investing in our Investment Portfolio.

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.

•

Investment Rating 1 represents a LMM portfolio company that is performing in a manner which significantly exceeds expectations.



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Investment Rating 2 represents a LMM portfolio company that, in general, is performing above expectations.



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Investment Rating 3 represents a LMM portfolio company that is generally performing in accordance with expectations.



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Investment Rating 4 represents a LMM portfolio company that is underperforming expectations. Investments with such a rating require increased monitoring and scrutiny by us.



•

Investment Rating 5 represents a LMM portfolio company that is significantly underperforming. Investments with such a rating require heightened levels of monitoring and scrutiny by us and involve the recognition of significant unrealized depreciation on such investment.

        All new LMM portfolio investments receive an initial Investment Rating of 3.

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        The following table shows the distribution of our LMM portfolio investments on the 1 to 5 investment rating scale at fair value as of September 30, 2016 and December 31, 2015:

        Based upon our investment rating system, the weighted-average rating of our LMM portfolio was approximately 2.3 as of September 30, 2016 and 2.2 as of December 31, 2015.

        As of September 30, 2016, our total Investment Portfolio had five investments on non-accrual status, which comprised approximately 0.4% of its fair value and 2.8% of its cost. As of December 31, 2015, our total Investment Portfolio had six investments on non-accrual status, which comprised approximately 0.4% of its fair value and 3.7% 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 and to an increase in defaults on our debt investments and to difficulty in maintaining historical dividend payment rates 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.

DISCUSSION AND ANALYSIS OF RESULTS OF OPERATIONS

Comparison of the three months ended September 30, 2016 and September 30, 2015

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Investment Income

        For the three months ended September 30, 2016, total investment income was $46.6 million, a 9% increase over the $42.6 million of total investment income for the corresponding period of 2015. This comparable period increase was principally attributable to (i) a $1.4 million increase in interest income primarily related to higher average levels of portfolio debt investments and (ii) a $2.8 million increase in dividend income from Investment Portfolio equity investments. The $4.0 million increase in total investment income in the three months ended September 30, 2016 includes the impact of an increase of $0.3 million primarily related to higher accelerated prepayment and repricing activity for certain Investment Portfolio debt investments when compared to the same period in 2015 and an increase of $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 during the period when compared to the same period in 2015.

Expenses

        For the three months ended September 30, 2016, total expenses increased to $16.0 million from $14.7 million for the corresponding period of 2015. This comparable period increase in operating expenses was principally attributable to (i) a $0.6 million increase in compensation expense related to increases in the number of personnel, base compensation levels and incentive compensation accruals, (ii) a $0.5 million increase in share-based compensation expense and (iii) a $0.3 million increase in interest expense, primarily due to an increase in interest expense on the Credit Facility in the three months ended September 30, 2016, with these increases partially offset by a $0.1 million increase in the expenses allocated to the External Investment Manager (see further discussion in "Overview"), in each case when compared to the same period in the prior year. For the three months ended September 30, 2016, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.5% on an annualized basis, compared to 1.3% on an annualized basis for the three months ended September 30, 2015 and 1.4% for the year ended December 31, 2015.

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Net Investment Income

        Net investment income for the three months ended September 30, 2016 was $30.6 million, or a 10% increase, compared to net investment income of $27.9 million for the corresponding period of 2015. The increase in net investment income was principally attributable to the increase in total investment income, partially offset by higher operating expenses as discussed above.

Distributable Net Investment Income

        For the three months ended September 30, 2016, distributable net investment income increased 11% to $32.7 million, or $0.62 per share, compared with $29.5 million, or $0.59 per share, in the corresponding period of 2015. The increase in distributable net investment income was primarily due to the higher level of total investment income, partially offset by higher operating expenses as discussed above. Distributable net investment income on a per share basis for the three months ended September 30, 2016 reflects (i) an increase of approximately $0.01 per share from the comparable period in 2015 attributable to the net increase in the comparable levels of accelerated prepayment and repricing activity for certain Investment Portfolio debt investments, (ii) an increase of approximately $0.03 per share from the comparable period in 2015 attributable to the increase in dividend income that is considered to be less consistent on a recurring basis or non-recurring and (iii) a greater number of average shares outstanding compared to the corresponding period in 2015 primarily due to shares issued through the ATM Program (as defined in "—Liquidity and Capital Resources—Capital Resources" below) and shares issued pursuant to our restricted stock plan and dividend reinvestment plan.

Net Increase in Net Assets Resulting from Operations

        The net increase in net assets resulting from operations during the three months ended September 30, 2016 was $43.2 million, or $0.82 per share, compared with $20.7 million, or $0.41 per share, during the three months ended September 30, 2015. This $22.5 million increase from the same period in the prior year period was primarily the result of (i) a $16.9 million increase in net change in unrealized appreciation (depreciation) from net unrealized depreciation of $9.1 million for the three months ended September 30, 2015 to net unrealized appreciation of $7.8 million for the three months ended September 30, 2016, (ii) a $5.6 million increase in the net realized gain (loss) from investments from a net realized loss of $1.3 million during the three months ended September 30, 2015 to a net realized gain of $4.3 million for the three months ended September 30, 2016 and (iii) a $2.7 million increase in net investment income as discussed above, partially offset by a $2.7 million decrease in the income tax benefit for the three months ended September 30, 2016. The net realized gain of $4.3 million for the three months ended September 30, 2016 was primarily the result of (i) the net realized gain on the exit of three LMM investments totaling $13.2 million and (ii) the net realized gain of $1.2 million due to activity in our Other Portfolio, partially offset by (i) the realized loss of $7.3 million on the exit of a Private Loan investment and (ii) the realized loss of $2.6 million related to the restructuring of a Middle Market investment.

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        The following table provides a summary of the total net unrealized appreciation of $7.8 million for the three months ended September 30, 2016:

        The income tax benefit for the three months ended September 30, 2016 of $0.5 million consisted of a deferred tax benefit of $1.4 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, partially offset by $0.9 million of other current tax expense primarily related to an accrual for excise tax on our estimated undistributed taxable income.

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Comparison of the nine months ended September 30, 2016 and September 30, 2015

 

Investment Income

        For the nine months ended September 30, 2016, total investment income was $131.5 million, a 9% increase over the $121.1 million of total investment income for the corresponding period of 2015. This comparable period increase was principally attributable to (i) a $4.2 million increase in interest income primarily related to higher average levels of portfolio debt investments and (ii) a $7.7 million increase in dividend income from Investment Portfolio equity investments, partially offset by (i) a $0.8 million decrease in fee income and (ii) a $0.7 million decrease in investment income from Marketable

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securities and idle funds investments. The $10.4 million increase in total investment income in the nine months ended September 30, 2016 includes an increase of $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 during the period when compared to the same period in 2015 and the impact of a decrease of $0.9 million primarily related to lower accelerated prepayment and repricing activity for certain Investment Portfolio debt investments when compared to the same period in 2015.

Expenses

        For the nine months ended September 30, 2016, total expenses increased to $46.1 million from $42.5 million for the corresponding period of 2015. This comparable period increase in operating expenses was principally attributable to (i) a $1.4 million increase in share-based compensation expense, (ii) a $1.3 million increase in interest expense, primarily due to an increase in interest expense on the Credit Facility generally due to the higher average balance outstanding in the nine months ended September 30, 2016, (iii) a $1.0 million increase in compensation expense related to increases in the number of personnel, base compensation levels and incentive compensation accruals and (iv) a $0.5 million increase in general and administrative expenses, with these increases partially offset by a $0.6 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 nine months ended September 30, 2016, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.4% on an annualized basis, which is consistent with the ratio on an annualized basis for the nine months ended September 30, 2015 and for the year ended December 31, 2015.

Net Investment Income

        Net investment income for the nine months ended September 30, 2016 was $85.4 million, or a 9% increase, compared to net investment income of $78.6 million for the corresponding period of 2015. The increase in net investment income was principally attributable to the increase in total investment income, partially offset by higher operating expenses as discussed above.

Distributable Net Investment Income

        For the nine months ended September 30, 2016, distributable net investment income increased 10% to $91.3 million, or $1.77 per share, compared with $83.1 million, or $1.71 per share, in the corresponding period of 2015. 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 nine months ended September 30, 2016 reflects (i) a decrease of approximately $0.02 per share from the comparable period in 2015 attributable to the net decrease in the comparable levels of accelerated prepayment and repricing activity for certain Investment Portfolio debt investments, (ii) an increase of approximately $0.03 per share from the comparable period in 2015 attributable to the increase in dividend income that is considered to be less consistent on a recurring basis or non-recurring and (iii) a greater number of average shares outstanding compared to the corresponding period in 2015 primarily due to the March 2015 equity offering, shares issued through the ATM Program and shares issued pursuant to our restricted stock plan and dividend reinvestment plan.

Net Increase in Net Assets Resulting from Operations

        The net increase in net assets resulting from operations during the nine months ended September 30, 2016 was $90.9 million, or $1.76 per share, compared with $96.9 million, or $1.99 per share, during the nine months ended September 30, 2015. This $6.0 million decrease from the same period in the prior year period was primarily the result of (i) a $49.2 million decrease in net change in

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unrealized appreciation (depreciation) from net unrealized appreciation of $20.4 million for the nine months ended September 30, 2015 to net unrealized depreciation of $28.8 million for the nine months ended September 30, 2016 and (ii) a $6.0 million decrease in the income tax benefit from the same period in the prior year, partially offset by (i) a $6.8 million increase in net investment income as discussed above and (ii) a $42.4 million increase in the net realized gain (loss) from investments from a net realized loss of $9.0 million during the nine months ended September 30, 2015 to a net realized gain of $33.3 million for the nine months ended September 30, 2016. The net realized gain of $33.3 million for the nine months ended September 30, 2016 was primarily the result of (i) the net realized gain of $56.3 million on the exit five LMM investments and (ii) the net realized gain of $2.8 million due to activity in our Other Portfolio, partially offset by (i) the net realized loss of $9.6 million on the exit of three Private Loan investments, (ii) the net realized loss of $10.0 million related to the restructuring of three Middle Market investments, (iii) the net realized loss of $4.7 million on the exit of two Middle Market investments and (iv) the net realized loss of $1.6 million on the exit of a Marketable securities and idle funds investment.

        The following table provides a summary of the total net unrealized depreciation of $28.8 million for the nine months ended September 30, 2016:

        The income tax benefit for the nine months ended September 30, 2016 of $1.0 million principally consisted of a deferred tax benefit of $3.4 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, partially offset by other current tax expense related to (i) a $2.1 million accrual for excise tax on our estimated undistributed taxable income and (ii) other current tax expense of $0.3 million related to accruals for U.S. federal and state income taxes.

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Liquidity and Capital Resources

Cash Flows

        For the nine months ended September 30, 2016, we experienced a net increase in cash and cash equivalents in the amount of $11.5 million, which is the result of $9.4 million of cash provided by our operating activities and $2.0 million of cash provided by financing activities.

        During the period, we generated $9.4 million of cash from our operating activities, which resulted primarily from (i) cash flows we generated from the operating profits earned through our operating activities totaling $79.7 million, which is our $91.3 million of distributable net investment income, excluding the non-cash effects of the accretion of unearned income of $7.1 million, payment-in-kind interest income of $4.9 million, cumulative dividends of $1.5 million and the amortization expense for deferred financing costs of $1.9 million, (ii) cash uses totaling $423.5 million which primarily resulted from (a) the funding of new portfolio company investments and settlement of accruals for portfolio investments existing as of December 31, 2015, which collectively total $420.0 million, (b) $3.0 million related to decreases in payables and accruals and (c) $0.5 million from the purchase of Marketable securities and idle funds investments and (iii) cash proceeds totaling $353.2 million from (a) $347.9 million in cash proceeds from the sales and repayments of debt investments and sales of and return on capital of equity investments and (b) $4.3 million of cash proceeds from the sale of Marketable securities and idle funds investments and (c) decreases in other assets of $1.0 million.

        During the nine months ended September 30, 2016, $2.0 million in cash was provided by financing activities, which principally consisted of (i) $64.3 million in net cash proceeds from the ATM Program (described below), (ii) $22.0 million in net cash proceeds from the Credit Facility and (iii) $6.0 million in cash proceeds from issuance of SBIC debentures, partially offset by (i) $86.7 million in cash dividends paid to stockholders, (ii) $2.6 million for purchases of vested restricted stock from employees to satisfy their tax withholding requirements upon the vesting of such restricted stock and (iii) $1.0 million for payment of deferred loan costs, SBIC debenture fees and other costs.

Capital Resources

        As of September 30, 2016, we had $31.8 million in cash and cash equivalents and $242.0 million of unused capacity under the Credit Facility, which we maintain to support our investment and operating activities. As of September 30, 2016, our net asset value totaled $1,144.4 million, or $21.62 per share.

        The Credit Facility, which provides additional liquidity to support our investment and operational activities, includes total commitments of $555.0 million from a diversified group of fourteen lenders and matures in September 2020. The Credit Facility also contains an accordion feature which allows us to increase the total commitments under the facility to up to $750.0 million from new and existing lenders on the same terms and conditions as the existing commitments.

        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 (0.53% as of September 30, 2016) plus (i) 1.875% (or the applicable base rate (Prime Rate of 3.50% as of September 30, 2016) plus 0.875%) as long as we maintain an investment grade rating and meet certain agreed upon excess collateral and maximum leverage requirements, (ii) 2.0% (or the applicable base rate plus 1.0%) if we maintain an investment grade rating but, do not meet certain excess collateral and maximum leverage requirements or (iii) 2.25% (or the applicable base rate plus 1.25%) if we do not maintain an investment grade rating. 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

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interest coverage ratio of at least 2.0 to 1.0, (iii) maintaining an asset coverage ratio 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 2020, 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 September 30, 2016, we had $313.0 million in borrowings outstanding under the Credit Facility, the interest rate on the Credit Facility was 2.4% and we were in compliance with all financial covenants of the Credit Facility.

        Due to each of the Funds' status as a licensed SBIC, we have the ability to issue, through the Funds, debentures guaranteed by the SBA at favorable interest rates and favorable terms and conditions. In addition, in December 2015, the 2016 omnibus spending bill approved by Congress and signed into law by the President increased the amount of SBA-guaranteed debentures that affiliated SBIC funds can have outstanding from $225.0 million to $350.0 million. This new legislation allows us to issue additional SBIC debentures, subject to SBA approval, above the $225.0 million that we had outstanding prior to the legislation. In August 2016, we received a license from the SBA to form and operate a third SBIC, which at the time provided us with up to an additional $125.0 million of additional long-term, fixed interest rate debt capital through the issuance of SBA-guaranteed debentures. During September 2016, we issued $6.0 million of SBIC debentures, leaving $119.0 million of remaining capacity. 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 semi-annually. 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. On September 30, 2016, through our three wholly owned SBICs, we had $231.0 million of outstanding SBIC debentures guaranteed by the SBA, which bear a weighted-average annual fixed interest rate of approximately 4.1%, paid semi-annually, and mature ten years from issuance. The first maturity related to our SBIC debentures does not occur until 2017, and the weighted-average remaining duration is approximately 5.0 years as of September 30, 2016.

        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 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 6.125% Notes; 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 6.125% Notes mature on April 1, 2023, and may be redeemed in whole or in part at any time or from time to time at our option on or after April 1, 2018. We may from time to time repurchase 6.125% Notes in accordance with the 1940 Act and the rules promulgated thereunder. As of September 30, 2016, the outstanding balance of the 6.125% Notes was $90.7 million.

        The indenture governing the 6.125% Notes (the "6.125% Notes 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 6.125% Notes and the Trustee if we cease to be subject to the reporting requirements of the Securities Exchange Act of 1934. These covenants are subject to limitations and exceptions that are described in the 6.125% Notes Indenture.

        In November 2014, we issued $175.0 million in aggregate principal amount of the 4.50% Notes at an issue price of 99.53%. The 4.50% Notes 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; effectively subordinated to all of our existing and future

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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 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 bear interest at a rate of 4.50% per year payable semi-annually on June 1 and December 1 of each year, beginning June 1, 2015. We may from time to time repurchase 4.50% Notes in accordance with the 1940 Act and the rules promulgated thereunder. As of September 30, 2016, the outstanding balance of the 4.50% Notes was $175.0 million.

        The indenture governing the 4.50% Notes (the "4.50% Notes 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 and the Trustee if we cease to be subject to the reporting requirements of the Securities Exchange Act of 1934. These covenants are subject to limitations and exceptions that are described in the 4.50% Notes Indenture.

        During March 2015, we completed a follow-on public equity offering of 4,370,000 shares of common stock, including the underwriters' full exercise of their option to purchase 570,000 additional shares, resulting in total net proceeds, including exercise of the underwriters' option to purchase additional shares and after deducting underwriting discounts and estimated offering expenses payable by us, of approximately $127.8 million.

        During November 2015, we commenced a program with 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 three months ended December 31, 2015, we sold 140,568 shares of our common stock at a weighted-average price of $31.98 per share and raised $4.5 million of gross proceeds under the ATM Program. Net proceeds were $4.3 million after commissions to the selling agents on shares sold and offering costs.

        During the nine months ended September 30, 2016, we sold 1,996,793 shares of our common stock at a weighted-average price of $32.67 per share and raised $65.2 million of gross proceeds under the ATM Program. Net proceeds were $64.3 million after commissions to the selling agents on shares sold and offering costs. As of September 30, 2016, sales transactions representing 30,804 shares had not settled and are not included in shares issued and outstanding on the face of the consolidated balance sheet, but are included in the weighted average shares outstanding on the consolidated statement of operations and in the shares used to calculate our net asset value per share. As of September 30, 2016, 362,639 shares were available for sale under the ATM Program.

        We anticipate that we will continue to fund our investment activities through existing cash and cash equivalents, the liquidation of Marketable securities and idle funds investments, 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. The composition of Marketable securities and idle funds investments will vary in a given period based upon, among other things, changes in market conditions, the underlying fundamentals in

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our Marketable securities and idle funds investments, our outlook regarding future LMM, Middle Market and Private Loan portfolio investment needs, and any regulatory requirements applicable to us.

        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 2016 annual meeting of stockholders because our common stock price per share had been trading significantly above the current 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%. 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 recent public equity and historical debt offerings, our $555.0 million Credit Facility, and the available leverage through the SBIC program, 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 ASU 2014-09, Revenue from Contracts with Customers (Topic 606). ASU 2014-9 supersedes the revenue recognition requirements under ASC Topic 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 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 new 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 new 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 May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606)—Narrow-Scope Improvements and Practical Expedients. This ASU clarified guidance on assessing collectability, presenting sales tax, measuring noncash consideration, and certain transition matters. The FASB tentatively decided to defer the effective date of the new revenue standard for public entities under U.S. GAAP for one year. If finalized, the new guidance will be effective for the annual reporting period beginning after December 15, 2017, including interim periods within that reporting period. Early adoption would be permitted for annual reporting periods beginning after December 15, 2016. We are currently evaluating the impact the adoption of this new accounting standard will have on our consolidated financial statements.

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        In May 2015, the FASB issued ASU 2015-07, Fair Value Measurements—Disclosures for Certain Entities that Calculate Net Asset Value per Share. This amendment updates guidance intended to eliminate the diversity in practice surrounding how investments measured at net asset value under the practical expedient with future redemption dates have been categorized in the fair value hierarchy. Under the updated guidance, investments for which fair value is measured at net asset value per share using the practical expedient should no longer be categorized in the fair value hierarchy, while investments for which fair value is measured at net asset value per share but the practical expedient is not applied should continue to be categorized in the fair value hierarchy. The updated guidance requires retrospective adoption for all periods presented and is effective for interim and annual reporting periods beginning after December 15, 2015, with early adoption permitted. The Company adopted this standard during the three months ended March 31, 2016. There was no impact of the adoption of this new accounting standard on our consolidated financial statements as none of our investments are measured through the use of the practical expedient.

        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 new guidance is effective for annual periods beginning after December 15, 2018, and interim periods therein. Early application is permitted. The impact of the adoption of this new accounting standard on our consolidated financial statements is currently being evaluated.

        In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation: Improvements to Employee Share-Based Payment Accounting, which is intended to simplify several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The guidance is effective for annual periods beginning after December 15, 2016, and interim periods therein. Early application is permitted. The Company elected to early adopt this standard during the three months ended March 31, 2016. See further discussion of the impact of the adoption of this standard in "Note B.8.—Summary of Significant Accounting Policies—Share-based Compensation" in the notes to 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. Early application is permitted. The impact of the adoption of this new accounting standard on our consolidated financial statements is not expected to be material.

        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 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.

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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 September 30, 2016, we had a total of $108.8 million in outstanding commitments comprised of (i) 30 investments with commitments to fund revolving loans that had not been fully drawn or term loans with additional commitments not yet funded and (ii) nine investments with equity capital commitments that had not been fully called.

Contractual Obligations

        As of September 30, 2016, the future fixed commitments for cash payments in connection with our SBIC debentures, the 4.50% Notes and the 6.125% Notes for each of the next five years and thereafter are as follows:

        As of September 30, 2016, we had $313.0 million in borrowings outstanding under our Credit Facility, and the Credit Facility is currently scheduled to mature in September 2020. The Credit Facility contains two, one-year extension options which could extend the maturity to September 2022. 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 September 30, 2016, we had a receivable of $2.2 million due from the External Investment Manager which included approximately $1.4 million primarily related to operating expenses incurred by us required to support the External Investment Manager's business, along with dividends declared but not paid by the External Investment Manager of approximately $0.8 million.

        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

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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 September 30, 2016, $2.0 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, $1.7 million was deferred into phantom Main Street stock units, representing 55,753 shares of our common stock. Including phantom stock units issued through dividend reinvestment, the phantom stock units outstanding as of September 30, 2016 represented 63,257 shares of our common stock. Any amounts deferred under the plan represented by phantom stock units will not be issued or included as outstanding on the consolidated statement 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 on our consolidated statement of operations as earned.

PLAN OF DISTRIBUTION

        We have entered into separate equity distribution agreements, each dated November 18, 2016, with each of Goldman, Sachs & Co., Raymond James & Associates, Inc., RBC Capital Markets, LLC and Robert W. Baird & Co. Incorporated, under which each will act as our sales agent (each, a "Sales Agent" and, collectively, the "Sales Agents") in connection with the offer and sale of shares of our common stock pursuant to this prospectus supplement and the accompanying prospectus. Upon written instructions from us, a Sales Agent will use its commercially reasonable efforts consistent with its sales and trading practices to sell, as our sales agent, our common stock under the terms and subject to the conditions set forth in the respective equity distribution agreement. We will instruct each Sales Agent as to the amount of common stock to be sold by it. We may instruct the Sales Agent not to sell common stock if the sales cannot be effected at or above the price designated by us in any instruction. We or the Sales Agent may suspend the offering of shares of common stock upon proper notice and subject to other conditions.

        Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be "at the market," as defined in Rule 415 under the Securities Act, including sales made directly on the NYSE or similar securities exchange or sales made to or through a market maker other than on an exchange at prices related to the prevailing market prices or at negotiated prices.

        The Sales Agent will provide written confirmation of a sale to us no later than the opening of the trading day on the NYSE following each trading day in which shares of our common stock are sold under the equity distribution agreement. Each confirmation will include the number of shares of common stock sold on the preceding day, the net proceeds to us and the compensation payable by us to the Sales Agent in connection with the sales.

        Under the terms of the equity distribution agreements, Goldman, Sachs & Co., Raymond James & Associates, Inc., RBC Capital Markets, LLC and Robert W. Baird & Co. Incorporated will be entitled to compensation equal to up to 1.0% of the gross sales price of shares of our common stock sold through it as Sales Agent. We estimate that the total expenses for the offering, excluding compensation payable to the Sales Agents under the terms of each equity distribution agreement, will be approximately $250,000 (which includes up to $7,500 per fiscal quarter in reimbursement of the Sales Agents' aggregate reasonable legal fees and expenses of counsel).

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        Settlement for sales of shares of common stock will occur on the third trading day following the date on which such sales are made, or on some other date that is agreed upon by us and the Sales Agent in connection with a particular transaction, in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

        Under the terms of the equity distribution agreements, we also may sell shares of our common stock to the Sales Agents as principal for their own accounts at a price agreed upon at the time of sale. The Sales Agents may offer the common stock sold to them as principals from time to time through public or private transactions at market prices prevailing at the time of sale, at fixed prices, at negotiated prices, at various prices determined at the time of sale or at prices related to prevailing market prices. If we sell shares to a Sales Agent as principal, we will enter into a separate terms agreement with the applicable Sales Agent, setting forth the terms of such transaction, and we will describe the agreement in a separate prospectus supplement.

        We will report at least quarterly the number of shares of our common stock sold through the Sales Agents under the equity distribution agreements and the net proceeds to us.

        In connection with the sale of the common stock on our behalf, the Sales Agents may be deemed to be an "underwriter" within the meaning of the Securities Act, and the compensation of the Sales Agent may be deemed to be underwriting commissions or discounts. We have agreed to provide indemnification and contribution to the Sales Agents with respect to certain civil liabilities, including liabilities under the Securities Act.

        The offering of our shares of common stock pursuant to the equity distribution agreement will terminate upon the earlier of (i) the sale of all common stock subject to the equity distribution agreement or (ii) the termination of the equity distribution agreements as permitted therein.

Conflicts of Interest

        Affiliates of Goldman, Sachs & Co., Raymond James & Associates, Inc. and RBC Capital Markets, LLC, Sales Agents in this offering, act as lenders and/or agents under our Credit Facility. Certain of the net proceeds from the sale of our common stock, not including selling compensation, may be paid to such affiliates of Goldman, Sachs & Co., Raymond James & Associates, Inc. and RBC Capital Markets, LLC in connection with the repayment of debt owed under our Credit Facility. As a result, Goldman, Sachs & Co., Raymond James & Associates, Inc. and RBC Capital Markets, LLC and/or their affiliates may receive more than 5% of the net proceeds of this offering, not including selling compensation.

        The Sales Agents and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial and non-financial activities and services. The Sales Agents and/or their affiliates from time to time provide and may in the future provide investment banking, commercial banking and financial advisory services to us, for which they have received and may receive customary compensation.

        In the ordinary course of their various business activities, the Sales Agents and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively traded securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to our assets, securities and/or instruments (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with us. The Sales Agents and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time

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hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments. In addition, the Sales Agents and/or their affiliates may from time to time refer investment banking clients to us as potential portfolio investments. If we invest in those clients, we may utilize net proceeds from this offering to fund such investments, and the referring Sales Agent or its affiliate may receive placement fees from its client in connection with such financing, which placement fees may be paid out of the amount funded by us.

        The addresses of the Sales Agents are: Goldman, Sachs & Co., 200 West Street, New York, NY 10282; Raymond James & Associates, Inc., 880 Carillon Parkway, St. Petersburg, Florida 33716; RBC Capital Markets, LLC, Three World Financial Center, 8th Floor, 200 Vesey Street, New York, NY 10281; and Robert W. Baird & Co. Incorporated, 777 East Wisconsin Avenue, Milwaukee, Wisconsin 53202.

LEGAL MATTERS

        Certain legal matters regarding the shares of common stock offered hereby will be passed upon for us by Sutherland Asbill & Brennan LLP, Washington D.C., and certain legal matters in connection with this offering will be passed upon for the Sales Agents by Fried, Frank, Harris, Shriver & Jacobson LLP, New York, New York.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The audited consolidated financial statements, financial highlights, Schedule 12-14 and the schedule of Senior Securities of Main Street Capital Corporation, included in this prospectus supplement and elsewhere in the registration statement have been so included in reliance upon the reports of Grant Thornton LLP, independent registered public accountants, as stated in their reports appearing herein. Grant Thornton LLP's principal business address is Grant Thornton Tower, 171 North Clark, Suite 200, Chicago, Illinois, 60601.

AVAILABLE INFORMATION

        We have filed with the SEC a universal shelf registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to our shares of common stock offered by this prospectus supplement. The registration statement contains additional information about us and our shares of common stock being offered by this prospectus supplement.

        We file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Securities Exchange Act of 1934. You may inspect and copy these reports, proxy statements and other information, as well as the registration statement and related exhibits and schedules, at the Public Reference Room of the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an Internet site that contains reports, proxy and information statements and other information filed electronically by us with the SEC, which are available on the SEC's website at www.sec.gov. Copies of these reports, proxy and information statements and other information may be obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC's Public Reference Section, 100 F Street, N.E., Washington, D.C. 20549.

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INTERIM FINANCIAL STATEMENTS

MAIN STREET CAPITAL CORPORATION

Consolidated Balance Sheets

(dollars in thousands, except shares and per share amounts)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Operations

(dollars in thousands, except shares and per share amounts)

(Unaudited)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Changes in Net Assets

(dollars in thousands, except shares)

(Unaudited)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Cash Flows

(dollars in thousands)

(Unaudited)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Schedule Of Investments

September 30, 2016

(dollars in thousands)

(Unaudited)

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MAIN STREET CAPITAL CORPORATION

Consolidated Schedule Of Investments (Continued)

September 30, 2016

(dollars in thousands)

(Unaudited)