Filed Pursuant to Rule 424(b)(3)
Registration No. 333-228575

MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT

Dear Community 1^st Bank Shareholder:

On October 11, 2018, First Interstate BancSystem, Inc. (which we refer to as “First Interstate”), First Interstate Bank and Community 1^st Bank (which we refer to as “CMYF”) entered into an Agreement and Plan of Merger (which we refer to as the “merger agreement”) under which CMYF will merge with and into First Interstate Bank, with First Interstate Bank remaining as the surviving entity. This transaction is referred to in this document as the “merger.” Before the merger can be completed, the shareholders of CMYF must approve the merger agreement.

If the merger is completed, CMYF shareholders will be entitled to receive 0.3784 shares of First Interstate Class A common stock for each share of CMYF common stock they own. The maximum number of shares of First Interstate Class A common stock estimated to be issuable upon completion of the merger is 492,069. Based on First Interstate’s closing price of $44.62 on October 11, 2018, which was the last trading date preceding the public announcement of the proposed merger, each share of CMYF common stock exchanged for 0.3784 shares of First Interstate Class A common stock would have a value of $16.88, or approximately $21.0 million in the aggregate. Based on First Interstate’s closing price of $38.92 on January 31, 2019, which is the most recent practicable trading day before the printing of this document, each share of CMYF common stock exchanged for 0.3784 shares of First Interstate Class A common stock would have a value of $14.73. The common stock of First Interstate trades on the Nasdaq Global Select Market under the symbol “FIBK.” The Class A common stock of CMYF trades on the OTC Market’s Pink Market Place under the symbol “CMYF.” The market price of both First Interstate Class A common stock and CMYF common stock will fluctuate before the completion of the merger. Therefore, you are urged to obtain current market quotations for First Interstate Class A common stock and CMYF common stock.

Although the number of shares of First Interstate Class A common stock that holders of CMYF common stock will be entitled to receive is fixed, the market value of the stock consideration will fluctuate with the market price of First Interstate Class A common stock and will not be known at the time CMYF shareholders vote on the merger. CMYF has the right to terminate the merger agreement if, at any time during a five-day period commencing on the fifth day before closing, the average closing price of First Interstate Class A common stock over the 20 consecutive trading days ending on and including the fifth day before closing (1) is less than $36.54 and (2) underperforms the KBW Regional Banking Index by more than 20% during the same time period. If CMYF elects to exercise this termination right, then First Interstate has the option to increase the exchange ratio to a level that would eliminate the ability of CMYF to exercise this termination right.

The affirmative vote of two-thirds of the outstanding shares of CMYF common stock is required to approve the merger agreement. CMYF shareholders will vote to approve the merger agreement at a special meeting of shareholders to be held at 5:30 p.m., local time, on March 27, 2019 at 707 N. Post Street, Post Falls, Idaho 83854.

CMYF’s board of directors unanimously recommends that CMYF shareholders vote “FOR” the approval of the merger agreement.

This document contains information that you should consider in evaluating the proposed merger. In particular, you should carefully read the section captioned “Risk Factors” beginning on page 13 for a discussion of certain risk factors relating to the merger. You can also obtain information about First Interstate from documents filed with the Securities and Exchange Commission.

As always, we appreciate your support and look forward to seeing you at the shareholders’ meeting and we appreciate your continued support.

The shares of First Interstate Class A common stock to be issued in the merger are not deposits or savings accounts or other obligations of any bank or savings association and are not insured by the FDIC or any other governmental agency.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved the merger described in this document or the First Interstate Class A common stock to be issued in the merger, or passed upon the adequacy or accuracy of this document. Any representation to the contrary is a criminal offense.

The date of this proxy statement/prospectus is February 5, 2019, and it is first being mailed or otherwise delivered to shareholders of CMYF on or about February 7, 2019.

ABOUT THIS DOCUMENT

This proxy statement/prospectus, which we refer to as “this document,” forms part of a registration statement on Form S-4 filed with the Securities and Exchange Commission (which we refer to as the “SEC”) by First Interstate and constitutes a prospectus of First Interstate with respect to the shares of First Interstate Class A common stock to be issued to CMYF shareholders, as required by the merger agreement. This document also constitutes a proxy statement and a notice of meeting with respect to the special meeting of shareholders of CMYF.

You should only rely on the information contained in this document. No one has been authorized to provide you with information that is different from the information contained in this document. This document is dated February 5, 2019. You should not assume that the information contained in this document is accurate as of any date other than that date. Neither the mailing of this document to CMYF shareholders nor the issuance by First Interstate of its Class A common stock in connection with the merger will create any implication to the contrary.

This document does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Information contained in this document regarding First Interstate has been provided by First Interstate, and the information contained in this document regarding CMYF has been provided by CMYF.

COMMUNITY 1^st BANK

707 North Post Street

Post Falls, Idaho 83854

Notice of Special Meeting of Shareholders to be held on March 27, 2019

To the Shareholders of Community 1^st Bank:

Community 1^st Bank (which we refer to as “CMYF”) will hold a special meeting of shareholders (which we refer to as the “CMYF special meeting”) at 5:30 p.m., local time, on March 27, 2019, at 707 N. Post Street, Post Falls, Idaho 83854, to consider and vote on the following matters:

Both of these items are described in more detail in the accompanying proxy statement/prospectus and its annexes. We urge you to read these materials carefully and in their entirety. The enclosed document forms a part of this notice.

CMYF’s board of directors unanimously recommends that CMYF shareholders vote “FOR” both of the proposals.

CMYF shareholders of record as of the close of business on January 28, 2019 are entitled to notice of, and to vote at, the CMYF special meeting and any adjournments or postponements of the CMYF special meeting.

CMYF shareholders have the right to dissent from the merger and obtain payment of the cash appraisal fair value of their CMYF shares under applicable provisions of Idaho law. A copy of the provisions regarding dissenters’ rights is attached as Annex B to the accompanying proxy statement/prospectus. For details of your dissenter’s rights and how to exercise them, please see the discussion under “Description of the Merger—Dissenters’ Rights of Appraisal.”

Your vote is very important. Your proxy is being solicited by CMYF’s board of directors. For the proposed merger to be completed, the proposal to approve the merger agreement must be approved by the affirmative vote of at least two-thirds of the outstanding shares of CMYF common stock. The CMYF adjournment proposal will be approved if a majority of the votes cast on such proposal at the CMYF special meeting are voted in favor of such proposal.

Whether or not you plan to attend the CMYF special meeting, we urge you to vote. Shareholders of record may vote:

If you hold your stock in “street name” through a banker or broker, please follow the instructions on the voting instruction card furnished by the record holder.

If you have any questions or need assistance voting your shares, please contact Jerry C. Lyon, President and Operating Officer, at (208) 457-9610.

By Order of the Board of Directors,

/s/ William R. Basom

William R. Basom

Corporate Secretary

Post Falls, Idaho

February 5, 2019

REFERENCES TO AVAILABLE INFORMATION

This document incorporates important business and financial information about First Interstate from documents filed with the SEC that have not been included in or delivered with this document. You may read and copy these documents at the SEC’s public reference room located at 100 F Street, NE, Washington, DC 20549. Copies of these documents also can be obtained at prescribed rates by writing to the Public Reference Section of the SEC, at 100 F Street, NE, Washington, DC 20549 or by calling 1-800-SEC-0330 for additional information on the operation of the public reference facilities. This information is also available at the Internet site the SEC maintains at http://www.sec.gov. See “Where You Can Find More Information” on page 82.

You also may request orally or in writing copies of these documents at no cost by contacting First Interstate at:

First Interstate BancSystem, Inc.

401 North 31^st Street

Billings, Montana 59101

Attention: Kirk D. Jensen, General Counsel

Telephone: (406) 255-5304

If you are a CMYF shareholder and would like to request documents from First Interstate, please do so by March 20, 2019 to receive them before the CMYF special meeting.

TABLE OF CONTENTS

QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE CMYF SPECIAL MEETING

The following are answers to certain questions that you may have regarding the merger and the CMYF special meeting. We urge you to read carefully the remainder of this document because the information in this section may not provide all the information that might be important to you in determining how to vote. Additional important information is also contained in the annexes to, and the documents incorporated by reference in, this document.

To approve the merger agreement, CMYF has called a special meeting of its shareholders (which we refer to as the “CMYF special meeting”). This document also serves as a notice of the CMYF special meeting, and describes the proposals to be presented at the CMYF special meeting.

You should read this document carefully and in its entirety. The enclosed materials allow you to have your shares voted by proxy without attending the special meeting. Your vote is important. We encourage you to submit your proxy as soon as possible.

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First Interstate and First Interstate Bank entered into a merger agreement to acquire Idaho Independent Bank (“IIBK”) on the same date that it entered into the merger agreement with CMYF. The completion of CMYF’s merger is not conditioned upon or subject to the completion of the merger with IIBK (the “IIBK Merger”). However, since First Interstate and First Interstate Bank will be required to obtain the necessary regulatory approvals for two transactions rather than one, it is possible that the IIBK Merger will lengthen the amount of time that would otherwise be needed to obtain all regulatory approvals of the merger if CMYF were the only banking institution being acquired by First Interstate and First Interstate Bank. See “Risk Factors—Risks Related to the Merger—First Interstate and First Interstate Bank are also acquiring IIBK in the IIBK Merger and the conditions to completing that merger may result in delays in completing the CMYF merger, or make it more difficult or time consuming than expected” for a discussion of the impact that the IIBK acquisition would have on First Interstate’s and First Interstate Bank’s business and operations if both the CMYF and IIBK acquisitions are completed.

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Approval of the CMYF adjournment proposal requires that the votes cast in favor of the proposal exceed the votes cast against. Abstentions and broker non-votes will not affect the outcome of the vote on the CMYF adjournment proposal.

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If you hold your stock in “street name” through a banker or broker, please refer to the specific instructions set forth on the proxy or voting instruction card.

If you hold your shares of CMYF common stock in “street name” through a bank or broker, you should contact your bank or broker to change your vote or revoke your proxy.

Your last vote will be the vote that is counted.

If your shares are held in “street name,” you must vote your shares according to the voting instruction form by the deadline set by your broker, bank or other nominee.

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You should read “Description of the Merger—Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 49 of this document for more information about the United States federal income tax consequences of the merger. Tax matters can be complicated and the tax consequences of the merger will depend on your particular tax situation. You should consult your tax advisor to determine the tax consequences of the merger to you.

Jerry C. Lyon

President and Chief Operating Officer

Community 1^st Bank

707 N. Post Street

Post Falls, Idaho 83854

(208) 457-9610

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RISK FACTORS

In deciding how to vote, you should consider carefully all of the information included in this document and its annexes, all of the information incorporated by reference into the document and the risk factors identified by First Interstate with respect to First Interstate’s operations included in its filings with the SEC, including First Interstate’s Annual Report on Form 10-K for the year ended December 31, 2017. See “Where You Can Find More Information.” In addition, you should consider the following risk factors.

Risks Related to the Merger

Because the price of First Interstate Class A common stock will fluctuate, CMYF shareholders cannot be certain of the market value of the merger consideration.

Upon the completion of the merger, each share of CMYF common stock outstanding immediately before the completion of the merger will be converted into the right to receive 0.3784 shares of First Interstate Class A common stock. The market value of the merger consideration may vary from the closing price of First Interstate Class A common stock on the date the execution of the merger agreement was announced, on the date that this document was mailed to CMYF shareholders, on the date of the CMYF special meeting and on the date the merger is completed. The market price of First Interstate Class A common stock may fluctuate as a result of a variety of factors, including general market and economic conditions, changes in First Interstate’s business, operations and prospects, and regulatory considerations. Therefore, at the time of the CMYF special meeting, CMYF shareholders will not know or be able to calculate the market value of the First Interstate Class A common stock they will receive upon completion of the merger. For example, based on the range of closing prices of First Interstate Class A common stock during the period from October 11, 2018, the last trading day before public announcement of the merger, through January 31, 2019, the last practicable date before the date of this document, the merger consideration represented a market value ranging from a low of $13.10 to a high of $16.88 for each share of CMYF common stock. You should obtain current market quotations for shares of First Interstate Class A common stock and CMYF common stock.

The price of First Interstate Class A common stock might decrease after the merger.

Upon completion of the merger, holders of CMYF common stock will become shareholders of First Interstate. First Interstate Class A common stock could decline in value after the merger. For example, during the twelve-month period ending on January 31, 2019 (the most recent practicable date before the printing of this document), the closing price of First Interstate Class A common stock varied from a low of $35.03 to a high of $46.95 and ended that period at $38.92. The market value of First Interstate Class A common stock fluctuates based upon general market conditions, First Interstate’s business, operations and prospects and other factors. Further, the market price of First Interstate Class A common stock after the merger may be affected by factors different from those currently affecting the common stock of First Interstate or CMYF. The businesses of CMYF and First Interstate Bank differ and, accordingly, the results of operations of the combined company and the market price of First Interstate’s shares of common stock following the merger may be affected by factors different from those currently affecting the independent results of operations and market prices of common stock of each of CMYF and First Interstate. For a discussion of the business of First Interstate, First Interstate Bank and of certain factors to consider in connection with First Interstate’s business, see the documents incorporated by reference in this document and referred to under “Where You Can Find More Information” beginning on page 82.

CMYF, First Interstate Bank and First Interstate will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effects of the merger on employees and customers may have an adverse effect on CMYF, First Interstate Bank or First Interstate. These uncertainties may impair CMYF’s, First Interstate Bank’s or First Interstate’s ability to attract, retain and motivate key personnel until the merger is completed, and could

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cause customers and others that interact with CMYF, First Interstate Bank or First Interstate to seek to change existing business relationships with CMYF, First Interstate Bank or First Interstate. Retention of certain employees by CMYF, First Interstate Bank or First Interstate may be challenging while the merger is pending, as certain employees may experience uncertainty about their future roles with the combined company. If key employees depart because of issues relating to the uncertainty and difficulty of integration, or a desire not to remain with CMYF or First Interstate, CMYF’s business or First Interstate’s business could be harmed. In addition, subject to certain exceptions, CMYF has agreed to operate its business in the ordinary course before closing, which may prevent it from pursuing certain growth opportunities. See “Description of the Merger—Conduct of Business Before the Merger” for a description of the restrictive covenants applicable to CMYF, First Interstate and First Interstate Bank.

Failure to complete the merger could negatively impact the stock prices and future businesses and financial results of First Interstate, First Interstate Bank and CMYF.

There can be no assurance that the merger will be completed. If the merger is not completed, the ongoing businesses of First Interstate. First Interstate Bank and CMYF may be adversely affected and First Interstate, First Interstate Bank and CMYF will be subject to a number of risks, including the following:

In addition, if the merger is not completed, First Interstate, First Interstate Bank and/or CMYF may experience negative reactions from the financial markets and from their respective customers and employees. First Interstate, First Interstate Bank and/or CMYF also could be subject to litigation related to any failure to complete the merger or to proceedings commenced by First Interstate and First Interstate Bank on the one hand, or CMYF on the other hand, against the other seeking damages or to compel the other to perform its obligations under the merger agreement. These factors and similar risks could have an adverse effect on the results of operation, business and stock prices of First Interstate, First Interstate Bank and CMYF.

First Interstate and First Interstate Bank may be unable to successfully integrate CMYF’s operations or retain CMYF’s employees, which could adversely affect the combined company.

The merger involves the integration of two banks that have previously operated independently. The difficulties of combining the operations of the two banks include, among other things: integrating personnel with diverse business backgrounds; combining different corporate cultures; and retaining key employees.

The process of integrating operations could cause an interruption of, or loss of momentum in, the activities of the business and the loss of key personnel. The integration of the two banks will substantially benefit from the experience and expertise of certain key employees of CMYF who are expected to be retained by First Interstate Bank. First Interstate Bank may not be successful in retaining these employees for the time period necessary to successfully integrate CMYF’s operations with those of First Interstate Bank. The diversion of management’s attention and any delays or difficulties encountered in connection with the merger and the integration of the two companies’ operations could have an adverse effect on the business and results of operations of First Interstate Bank following the merger.

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Additionally, First Interstate and First Interstate Bank may not be able to successfully achieve the level of cost savings, revenue enhancements and other synergies that it expects, and may not be able to capitalize upon the existing customer relationships of CMYF to the extent anticipated, or it may take longer, or be more difficult or expensive than expected, to achieve these goals. These matters could have an adverse effect on First Interstate’s business, results of operation and stock price.

First Interstate and First Interstate Bank are also acquiring IIBK in the IIBK Merger and the conditions to completing that merger may result in delays in completing the CMYF merger, or make it more difficult or time consuming than expected.

First Interstate and First Interstate Bank announced the proposed acquisition of IIBK in the IIBK Merger on the same date as the CMYF merger. The IIBK Merger is subject to customary closing conditions, including receipt of regulatory approvals for the IIBK Merger, as well as the approval of the shareholders of IIBK. The preparation and filing of regulatory applications, and a separate prospectus/proxy statement, as well as planning the integration of CMYF at the same time as obtaining the regulatory approvals and planning the integration of CMYF is time consuming and could divert management attention and resources away from the CMYF merger. It is possible that because of the complexities involved in acquiring two financial institutions simultaneously, First Interstate and First Interstate Bank could experience delays in regulatory approval. This could result in the merger process taking longer than anticipated, which could be costly to First Interstate, First Interstate Bank and CMYF, disrupt First Interstate’s or CMYF’s ongoing businesses relationships with their respective clients, customers, depositors and employees or result in a failure to achieve the anticipated benefits of the CMYF merger.

The termination fee and the restrictions on solicitation contained in the merger agreement may discourage other companies from trying to acquire CMYF.

Until the completion of the merger, with some exceptions, CMYF is prohibited from soliciting, initiating, encouraging or participating in any discussion of or otherwise considering any inquiry or proposal that may lead to an acquisition proposal, such as a merger or other business combination transactions, with any person other than First Interstate and First Interstate Bank. In addition, CMYF has agreed to pay an $808,624 termination fee to First Interstate in specified circumstances. These provisions could discourage other companies that may have an interest in acquiring CMYF from considering or proposing such an acquisition even though those other companies might be willing to offer greater value to CMYF’s shareholders than First Interstate has offered in the merger. The payment of the termination fee would also have a material adverse effect on CMYF’s financial condition and results of operations.

Certain of CMYF’s directors and officers have interests that are different from, or in addition to, interests of CMYF shareholders generally.

Certain executive officers and directors of CMYF have interests in the merger that are different from, or in addition to, the interests of CMYF shareholders generally. These include: (1) a cash payment in connection with the termination of all outstanding CMYF stock options; and (2) provisions in the merger agreement relating to indemnification of directors and officers and insurance for directors and officers of CMYF for events occurring before the merger. For a more detailed discussion of these interests, see “Description of the Merger—Interests of Certain Persons in the Merger that are Different from Yours” beginning on page 53.

Both First Interstate and CMYF shareholders will have a reduced ownership and voting interest after the merger and will exercise less influence over management of the combined organization.

First Interstate shareholders and CMYF shareholders each currently have the right to vote in the election of their respective board of directors and on various other matters affecting their respective company. Upon the completion of the merger, CMYF’s shareholders will become shareholders of First Interstate with a percentage ownership of the combined organization that is substantially smaller than such shareholders’ percentage

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ownership of CMYF. Following completion of the merger, CMYF shareholders will own approximately 1.2% of the outstanding shares of First Interstate Class A common stock and 0.8% of the aggregate outstanding shares of the First Interstate common stock, which equates to 0.3% of the voting power of First Interstate common stock. Additionally, upon the completion of the IIBK Merger, CMYF’s shareholders percentage ownership of First Interstate will further decrease.

The reduced voting power of CMYF shareholders is further effected by the two classes of common stock that First Interstate maintains. First Interstate Class B common stock is entitled to five votes per share, while shares of First Interstate Class A common stock, which is what will be issued to CMYF shareholders, are entitled to one vote per share. As of January 28, 2019, members of the Scott family held 21,285,014 shares of First Interstate Class B common stock and, therefore, controlled in excess of 71% of the voting power of First Interstate’s outstanding common stock. As a result, the Scott family will be able to exert a significant degree of influence or actual control over First Interstate’s management and affairs and over matters requiring shareholder approval, including the election of directors, a merger, consolidation or sale of all or substantially all of First Interstate’s assets and any other significant transaction. This concentrated control will limit CMYF shareholders’ future ability to influence corporate matters, and the interests of the Scott family may not always coincide with First Interstate’s interests or your interests.

The fairness opinion to the board of directors CMYF rendered on the date of the signing of the merger agreement does not reflect any changes in circumstances after the date of such fairness opinions.

D.A. Davidson delivered to the board of directors of CMYF its opinion on October 11, 2018. The opinion does not reflect changes that may occur or may have occurred after the date of such opinion, including changes to the operations and prospects of First Interstate or CMYF, changes in general market and economic conditions or regulatory or other factors that may materially alter or affect the value of First Interstate Class A common stock or CMYF common stock. The opinion speaks only as of the date on which it was rendered and not as of the date of this document or any other date.

There is no assurance when or even if the merger will be completed.

Completion of the merger is subject to satisfaction or waiver of a number of conditions. See “Description of the Merger—Conditions to Completing the Merger” beginning on page 55. There can be no assurance that First Interstate, First Interstate Bank and CMYF will be able to satisfy the closing conditions or that closing conditions beyond their control will be satisfied or waived.

First Interstate and CMYF can agree at any time to terminate the merger agreement, even if CMYF shareholders have already voted to approve the merger agreement. First Interstate and CMYF can also terminate the merger agreement under other specified circumstances.

Regulatory approvals may not be received, may take longer than expected, or may impose conditions that are not presently anticipated or that could have an adverse effect on the combined company following the merger.

Before the merger may be completed, First Interstate, First Interstate Bank and CMYF must obtain approvals from the Federal Reserve Board, the Montana Division and the Idaho Department, which approvals have been received. Other approvals, waivers or consents from regulators may also be required. In determining whether to grant these approvals the regulators consider a variety of factors, including the regulatory standing of each party. An adverse development in either party’s regulatory standing or other factors could result in an inability to obtain approval or delay their receipt. These regulators may impose conditions on the completion of the merger or require changes to the terms of the merger. Such conditions or changes could have the effect of delaying or preventing completion of the merger or imposing additional costs on or limiting the revenues of the combined company following the merger, any of which might have an adverse effect on the combined company following the merger.

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The shares of First Interstate Class A common stock to be received by CMYF shareholders as a result of the merger will have different rights from the shares of CMYF common stock.

Upon completion of the merger, CMYF shareholders will become First Interstate shareholders. Their rights as shareholders Class A common stock will be governed by Montana corporate law and the amended and restated articles of incorporation and bylaws of First Interstate. The rights associated with CMYF common stock are governed by Idaho law and the articles of incorporation and bylaws of CMYF and are different from the rights associated with First Interstate Class A common stock. See the section of this document entitled “Comparison of Rights of Shareholders” beginning on page 73 for a discussion of the different rights associated with First Interstate Class A common stock.

Goodwill incurred in the merger may negatively affect First Interstate’s financial condition.

To the extent that the merger consideration, consisting of shares of First Interstate Class A common stock to be issued in the merger, cash paid for fractional shares and cash paid to terminate CMYF options, exceeds the fair value of the net assets acquired in the merger, including identifiable intangibles of CMYF, that amount will be reported as goodwill by First Interstate. In accordance with current accounting guidance, goodwill will not be amortized but will be evaluated for impairment annually. A failure to realize expected benefits of the merger could adversely impact the carrying value of the goodwill recognized in the merger, and in turn negatively affect First Interstate’s financial condition.

Risks Relating to First Interstate’s Business

You should read and consider risk factors specific to First Interstate’s business that will also affect the combined company after the merger. These risks are described in the sections entitled “Risk Factors” in First Interstate’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 and in other documents incorporated by reference into this document. Please see the section entitled “Where You Can Find More Information” beginning on page 82 of this document for the location of information incorporated by reference into this document.

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CAUTIONARY STATEMENT ABOUT FORWARD-LOOKING STATEMENTS

Some of the statements contained or incorporated by reference in this document are forward looking statements within the meaning of the Private Securities Litigation Reform Act of 1995 giving First Interstate’s or CMYF’s expectations or predictions of future financial or business performance or conditions. Forward-looking statements are typically identified by words such as “believe,” “expect,” “anticipate,” “intend,” “target,” “estimate,” “continue,” “projections,” “prospects” or “potential,” by future conditional verbs such as “will,” “would,” “should,” “could” or “may,” or by variations of such words or by similar expressions. Such forward-looking statements include, but are not limited to, statements about the benefits of the merger, including future financial and operating results of First Interstate, CMYF or the combined entity following the merger, the combined entity’s plans, objectives, expectations and intentions, cost savings and/or revenue enhancements to be achieved in the merger, the expected timing of the completion of the merger, financing plans and the availability of capital, the likelihood of success and impact of litigation and other statements that are not historical facts. These statements are only predictions based on First Interstate’s and CMYF’s current expectations and projections about future events. There are important factors that could cause First Interstate’s and CMYF’s actual results, level of activity, performance or achievements to differ materially from the results, level of activity, performance or achievements expressed or implied by the forward-looking statements. In particular, you should consider the numerous risks and uncertainties described in the section entitled “Risk Factors” beginning on page 13.

These forward-looking statements are subject to numerous assumptions, risks, and uncertainties that change over time. In addition to factors previously disclosed in First Interstate’s reports filed with the SEC, the following factors, among others, could cause actual results to differ materially from forward-looking statements:

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Additionally, the timing and occurrence or non-occurrence of events may be subject to circumstances beyond First Interstate’s or CMYF’s control.

Annualized, projected and estimated numbers are used for illustrative purposes only, are not forecasts and may not reflect actual results.

For any forward-looking statements made in this document or in any documents incorporated by reference into this document, First Interstate and CMYF claim the protection of the safe harbor for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995. You are cautioned not to place undue reliance on these statements, which speak only as of the date of this document or the date of the applicable document incorporated by reference in this document. Except to the extent required by applicable law, neither First Interstate nor CMYF undertake to update forward-looking statements to reflect facts, circumstances, assumptions, or events that occur after the date the forward-looking statements are made. All written and oral forward-looking statements concerning the merger or other matters addressed in this document and attributable to First Interstate, CMYF, or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this document.

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SELECTED HISTORICAL FINANCIAL AND OTHER DATA

The following tables present selected historical financial information for First Interstate and for CMYF at and for the dates indicated. The following information is only a summary and not necessarily indicative of the results of future operations of First Interstate, CMYF or the combined company. The summary financial information for First Interstate is derived from prior filings made with the SEC, which are incorporated by reference into this document. The financial information for First Interstate at December 31, 2017 and 2016 and for the three years ended December 31, 2017 should be read in connection with the audited consolidated financial statements and related notes thereto included in its Annual Report on Form 10-K for the year ended December 31, 2017. The financial information for First Interstate for the nine months ended September 30, 2018 and 2017 should be read in connection with the unaudited financial statements and notes thereto included in its Quarterly Report on Form 10-Q for the period ended September 30, 2018. The results of operations for the nine months ended September 30, 2018 are not necessarily indicative of the operating results for the year ending December 31, 2018 or for any other period. See “Where You Can Find More Information” on page 82.

Selected Consolidated Historical Financial Data of First Interstate

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Selected Historical Financial Data of CMYF

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UNAUDITED COMPARATIVE PRO FORMA PER SHARE DATA

The following table shows information about First Interstate’s, CMYF’s and IIBK’s earnings per common share, dividends per share and book value per share, and similar information giving effect to the merger (which we refer to as “pro forma” information). In presenting the unaudited comparative pro forma information as of and for the periods shown, we assumed that First Interstate, CMYF and IIBK had been merged on the date indicated or at the beginning of the periods presented, as applicable.

The information listed as “per equivalent CMYF share” was obtained by multiplying the pro forma amounts by the exchange ratio of 0.3784. First Interstate anticipates that the combined company will derive financial benefits from the merger that include the opportunity to earn more revenue. The unaudited pro forma combined information, while helpful in illustrating the financial characteristics of First Interstate following the mergers under one set of assumptions, does not reflect these benefits and, accordingly, does not attempt to predict or suggest future results. The unaudited pro forma combined information also does not necessarily reflect what the historical results of First Interstate would have been had our companies been combined during this period.

The information in the following table is based on, and should be read together with, the historical financial information that we have presented in or incorporated by reference in this document.

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SPECIAL MEETING OF CMYF SHAREHOLDERS

This document is being provided to holders of CMYF common stock as CMYF’s proxy statement in connection with the solicitation of proxies by and on behalf of its board of directors to be voted at the CMYF special meeting and at any adjournment or postponement of the CMYF special meeting. This document is also being provided to holders of CMYF common stock as First Interstate’s prospectus in connection with the issuance by First Interstate of its shares of Class  A common stock as consideration in the proposed merger.

Date, Time and Place of Meeting

The CMYF special meeting is scheduled to be held as follows:

Date: March 27, 2019

Time: 5:30 p.m., local time

Place: 707 N. Post Street, Post Falls, Idaho 83854

Purpose of the Meeting

At the CMYF special meeting, CMYF’s shareholders will be asked to:

Who Can Vote at the Meeting

You are entitled to vote if the records of CMYF showed that you held shares of CMYF common stock as of the close of business on January 28, 2019, which is the record date for the CMYF special meeting. As of the close of business on the record date, 1,218,464 shares of CMYF common stock were outstanding. Each share of CMYF common stock has one vote on each matter presented to shareholders. If your shares are held in “street name” by your broker, bank or other nominee and you wish to vote in person at the CMYF special meeting, you will have to obtain a “legal proxy” from your broker, bank or other nominee entitling you to vote at the CMYF special meeting.

Quorum; Vote Required

The CMYF special meeting will conduct business only if a majority of the outstanding shares of CMYF common stock is represented in person or by proxy at the meeting to constitute a quorum. If you submit valid proxy instructions or attend the meeting in person, your shares will be counted to determine whether there is a quorum, even if you abstain from voting. If you fail to provide voting instructions to your broker, bank or other nominee with respect to a proposal, that broker, bank or other nominee will not vote your shares with respect to that proposal.

Approval of the merger agreement requires the affirmative vote of two-thirds of the outstanding shares of CMYF common stock. Failure to submit valid proxy instructions or to vote in person will have the same effect as a vote against the merger agreement. Broker non-votes and abstentions from voting will have the same effect as voting against the merger agreement.

Approval of the CMYF adjournment proposal requires that the votes cast in favor of the proposal exceed the votes cast against the proposal. The failure to vote in person or submit valid proxy instructions, broker non-votes and abstentions will have no effect on the voting on the proposal.

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Shares Held by CMYF Officers and Directors and by First Interstate

As of January 28, 2019, directors and executive officers of CMYF beneficially owned 496,034 shares of CMYF common stock. This equals 40.71% of the outstanding shares of CMYF common stock as of the January 28, 2019. Each of the directors and executive officers of CMYF, solely in their individual capacity as a CMYF shareholder, have entered into a separate voting agreement with First Interstate and First Interstate Bank to vote the 496,034 shares of CMYF common stock owned by them in favor of the merger agreement. As of the January 28, 2019, neither First Interstate nor any of its subsidiaries, directors or executive officers owned any shares of CMYF common stock.

Voting and Revocability of Proxies

You may vote in person at the CMYF special meeting or by proxy. To ensure your representation at the CMYF special meeting, CMYF recommends that you vote by proxy even if you plan to attend the CMYF special meeting. You can change your vote at the special meeting.

If you are a “shareholder of record,” you can vote your shares:

If you hold your stock in “street name” through a banker or broker, please refer to the specific instructions set forth on the proxy card.

CMYF shareholders whose shares are held in “street name” by their broker, bank or other nominee must follow the instructions provided by their broker, bank or other nominee to vote their shares. Your broker, bank or other nominee may allow you to deliver your voting instructions via telephone or the internet. If your shares are held in “street name” and you wish to vote in person at the CMYF special meeting, you will have to obtain a “legal proxy” from your broker, bank or other nominee entitling you to vote at the special meeting.

If you are a holder of record of CMYF common stock, voting instructions are included on the enclosed proxy card. If you properly complete and timely submit your proxy, your shares will be voted as you have directed. You may vote for, against or abstain with respect to each matter. If you are the holder of record of your shares of CMYF common stock and submit your proxy without specifying a voting instruction, your shares of CMYF common stock will be voted “FOR” the proposal to approve the merger agreement and “FOR” the CMYF adjournment proposal. If your shares are held in street name and you return an incomplete instruction card to your broker, bank or other nominee, that broker, bank or other nominee will not vote your shares with respect to any matter.

You may revoke your proxy at any time before it is voted at the special meeting by:

If your shares are held in “street name,” you should contact your broker, bank or other nominee to change your vote.

Attendance at the CMYF special meeting will not, in and of itself, constitute a revocation of a proxy. All written notices of revocation and other communication with respect to the revocation of proxies should be addressed to:

Community 1^st Bank

707 N. Post Street

Post Falls, Idaho 83854

Attention: William R. Basom, Corporate Secretary

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Solicitation of Proxies

CMYF will pay for the solicitation of proxies from CMYF shareholders. Additionally, directors, officers and employees of CMYF may solicit proxies personally and by telephone. None of these persons will receive additional or special compensation for soliciting proxies. CMYF will, upon request, reimburse brokers, banks and other nominees for their expenses in sending proxy materials to their customers who are beneficial owners and obtaining their voting instructions.

CMYF PROPOSAL NO. 1

APPROVAL OF THE MERGER AGREEMENT

At the CMYF special meeting, shareholders will consider and vote on a proposal to approve the merger agreement. Details about the merger agreement, including each party’s reasons for the merger, the effect of approval of the merger agreement and the timing of effectiveness of the merger, are discussed in the section entitled “Description of the Merger” beginning on page 27 of this document.

CMYF’s board of directors unanimously recommends

that CMYF shareholders vote “FOR”

approval of the merger agreement.

CMYF PROPOSAL NO. 2

ADJOURNMENT OF THE CMYF SPECIAL MEETING

If there are insufficient proxies at the time of the CMYF special meeting to approve the merger agreement, CMYF shareholders may be asked to vote on a proposal to adjourn the meeting to a later date to allow additional time to solicit additional proxies. CMYF’s board of directors does not currently intend to propose adjournment at the CMYF special meeting if there are sufficient votes to approve the merger agreement (Proposal No. 1).

CMYF’s board of directors unanimously recommends

that CMYF shareholders vote “FOR”

approval of the CMYF adjournment proposal.

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DESCRIPTION OF THE MERGER

The following summary of the merger agreement is qualified by reference to the complete text of the merger agreement. A copy of the merger agreement is attached as Annex A to this document and is incorporated by reference into this document. You should read the merger agreement completely and carefully as it, rather than this description, is the legal document that governs the merger.

General

The merger agreement provides for the merger of CMYF with and into First Interstate Bank, with First Interstate Bank as the surviving entity.

Pending Acquisition of Idaho Independent Bank

On October 11, 2018, First Interstate and First Interstate Bank also entered into an agreement to acquire IIBK. Pursuant to the IIBK merger agreement, IIBK will merge with and into First Interstate Bank, with First Interstate Bank as the surviving entity. The IIBK Merger was approved by the boards of directors of each of First Interstate, First Interstate Bank and IIBK and is expected to close in the first half of 2019. Completion of the IIBK Merger is subject to customary closing conditions, including receipt of required regulatory approvals and approval of IIBK’s shareholders. The completion of the CMYF merger is not conditioned upon or subject to the completion of the IIBK Merger. Under the terms of the IIBK merger agreement, holders of IIBK common stock will receive 0.50 shares of First Interstate Class A common stock for each share of IIBK common stock, which equates to an aggregate transaction value of approximately $180.5 million as of the date of the IIBK merger agreement.

At September 30, 2018, IIBK had total assets of $742.2 million, total deposits of $626.6 million and shareholders’ equity of $72.5 million.

For additional information on the IIBK Merger, including certain pro forma financial information, see “Unaudited Comparative Pro Forma Per Share Data.”

Consideration to be Received in the Merger

When the merger becomes effective, each share of CMYF common stock issued and outstanding immediately before completion of the merger will automatically be converted into the right to receive 0.3784 shares of First Interstate Class A common stock, plus cash in lieu of any fractional share, without interest.

If First Interstate declares a stock dividend or distribution on shares of its common stock or subdivides, splits, reclassifies or combines the shares of First Interstate Class A common stock before the effective time of the merger, then the exchange ratio will be adjusted to provide CMYF shareholders with the same economic effect as contemplated by the merger agreement before any of these events.

CMYF’s shareholders will not receive fractional shares of First Interstate Class A common stock. Instead, CMYF’s shareholders will receive a cash payment for any fractional shares in an amount equal to the product of (1) the fraction of a share of First Interstate Class A common stock to which he, she or it is entitled multiplied by (2) the average closing sales price of First Interstate Class A common stock over the 20 consecutive trading days ending on and including the fifth day before the closing date of the merger.

Background of the Merger

From time to time over the past several years, the board of directors of CMYF discussed and evaluated strategic alternatives to enhance shareholder value and achieve greater shareholder liquidity. During this period, CMYF’s board of directors has been focused on the challenges facing the community banking industry in general

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and, in some cases, CMYF specifically. Some of these challenges include the costs and burdens of the regulatory environment for community banks, the importance of scale to offset these costs, the competitive environment for loans and deposits and the desire for shareholder liquidity.

In October 2017, CMYF was approached by another community bank (“Party A”) that was interested in pursuing a potential merger with CMYF. CMYF and Party A had numerous discussions, but the verbal indication of pricing from Party A of approximately $12.60 per share in cash was determined to be too low by the CMYF board of directors on October 17, 2017.

At a meeting held on March 15, 2018, CMYF’s board of directors decided that it was in its shareholders’ best interest to engage D.A. Davidson as its financial advisor to assist the CMYF board of directors in reviewing and possibly pursuing its strategic alternatives. On March 19, 2018, CMYF engaged D.A. Davidson to evaluate the market landscape, identify potential merger partners and, if appropriate, assist the CMYF board of directors in pursuing and negotiating a transaction.

From March through April 2018, D.A. Davidson conducted due diligence on CMYF. D.A. Davidson contacted another community bank (“Party B”) that had expressed interest in CMYF at various points through the years about a possible merger. Party B was contacted due to its acquisition history, strong stock performance and ability to make a decision quickly on whether to pursue a merger or not. Party B and CMYF executed a mutual non-disclosure agreement without a “standstill” provision on March 22, 2018. Representatives from D.A. Davidson and CMYF’s Chief Executive Officer had an in-person meeting with the management team of Party B at Party B’s headquarters on May 2-3, 2018, which included material describing CMYF’s business and prospects. After several weeks of discussions, Party B elected not to pursue a potential transaction.

From May to late August 2018, D.A. Davidson contacted seven additional parties to determine if there was interest in a potential acquisition of CMYF and entered into confidentiality agreements without “standstill” provisions with four of those parties, including First Interstate.

D.A. Davidson made a presentation to the CMYF board of directors on June 19, 2018 to provide an update on the current market trends and discussions it had with potential partners to date.

CMYF held in-person meetings with three of the potential merger partners during the summer of 2018. The meetings consisted of management personnel from the three banks visiting CMYF and having meetings with CMYF’s Chief Executive Officer to discuss CMYF’s strategy, market opportunities and reasons for a possible sale.

In late August 2018, CMYF’s board of directors received two non-binding written letters of intent. CMYF’s board held a meeting on August 29, 2018 to review the letters of intent received from First Interstate and another party (“Party C”). The two letters of intent were discussed and compared to each other. The First Interstate letter of intent was for an all-stock transaction of 0.3874 shares of First Interstate Class A common stock for each share of CMYF common stock outstanding (with an implied value of $17.43 per share at such time). The letter of intent from Party C was for an all-cash transaction with an estimated value between $16.31 to $16.85 per CMYF common share, depending on CMYF’s future earnings and total transaction-related expenses. After discussion, it was determined to execute the letter of intent with First Interstate. On September 5, 2018, CMYF executed the First Interstate letter of intent dated August 31, 2018 and a separate exclusivity agreement with First Interstate.

On September 4, 2018, CMYF engaged the law firm of Breyer & Associates PC, which we refer to as Breyer & Associates, to be its legal counsel in connection with the proposed transaction.

During September and October 2018, CMYF set up an online data room containing due diligence materials, separately allowed First Interstate to conduct a loan file review and held several management meetings and conference calls with the executive officers of First Interstate. In September and October 2018, CMYF conducted reverse due diligence of First Interstate including information obtained from an online data room.

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On September 20, 2018, First Interstate circulated to CMYF a draft of the merger agreement and on September 27, 2018 circulated the ancillary agreements to the merger agreement through its counsel Luse Gorman, PC, which we refer to as Luse Gorman. Negotiation of the merger agreement took place between late September and early October 2018.

On September 26, 2018, there was a telephonic conference with CMYF’s Chairman and Chief Executive Officer, D.A. Davidson and Breyer & Associates to discuss the draft of the merger agreement.

On September 28, 2018, Breyer & Associates provided merger agreement comments to Luse Gorman.

On October 1, 2018, Breyer & Associates received a revised merger agreement from Luse Gorman. Also, on October 1, 2018, CMYF management had a reverse diligence conference call with the management team of First Interstate and Piper Jaffray & Co., which we refer to as Piper.

On October 4, 2018, at a CMYF board of directors meeting attended by Breyer & Associates and representatives of D.A. Davidson, the final terms of the definitive merger agreement were reviewed and discussed.

On October 5, 2018, CMYF provided its disclosure schedule to First Interstate and on October 8, 2018, First Interstate provided its disclosure schedule to CMYF.

On October 11, 2018, the board of directors of CMYF, together with Breyer & Associates and representatives of D.A. Davidson, met to consider approval of the merger agreement. At this meeting, D.A. Davidson reviewed the financial aspects of the proposed merger and rendered to the CMYF board of directors an opinion to the effect that, as of that date and subject to the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by D.A. Davidson as set forth in such opinion, the exchange ratio in the proposed merger was fair, from a financial point of view, to the shareholders of CMYF. Among other matters considered, the board of directors of CMYF reviewed the specific terms of the merger agreement, the form and value of the consideration to be received by CMYF shareholders, current market conditions including comparable bank merger and acquisition transactions, and the implications of the merger to CMYF’s employees, customers, and communities. After due consideration of these and other matters, the board of directors of CMYF determined that the proposed merger and merger agreement are advisable and in the best interests of CMYF’s shareholders, unanimously approved the merger agreement, and authorized the executive officers of CMYF to execute the merger agreement and take all necessary and appropriate action to consummate the transaction contemplated by the merger agreement.

On October 10, 2018, First Interstate’s board of directors held a meeting at which representatives of First Interstate’s senior management were present and representatives from its financial advisor, Piper, and its legal counsel, Luse Gorman, were present via telephone. Representatives of Piper reviewed the financial aspects of the transaction and the financial analysis supporting its fairness opinion. After discussion among First Interstate’s board of directors and its advisors, representatives of Piper delivered its opinion, dated October 10, 2018, to First Interstate’s board of directors to the effect that, as of such date and based on and subject to the qualifications, limitations and assumptions stated in its opinion, the merger consideration to be paid to the CMYF shareholders by First Interstate in the proposed merger was fair to First Interstate from a financial point of view. First Interstate’s board of directors was updated on the results of the completion of the legal and business due diligence review of CMYF. A representative of Luse Gorman reviewed in detail the terms of the merger agreement. After further discussion, First Interstate’s board of directors unanimously approved the merger agreement.

Later in the day on October 11, 2018, the parties executed the merger agreement and the parties to the ancillary agreements executed such agreements. On the afternoon of October 11, 2018, the parties issued a joint press release announcing the proposed transaction.

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CMYF’s Reasons for the Merger and Recommendation of the Board of Directors

The CMYF board of directors believes the merger is in the best interests of CMYF and its shareholders. After careful consideration, the CMYF board of directors unanimously approved the merger agreement at a meeting held on October 11, 2018, and recommends that CMYF shareholders vote “FOR” approval of the merger agreement and “FOR” the CMYF adjournment proposal.

In reaching its determination to approve the merger agreement, the CMYF board of directors consulted with CMYF’s management and its financial and legal advisors, and considered a number of factors, including the following material factors:

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In the course of its deliberations regarding the merger, the CMYF board of directors also considered the following potential risks and uncertainties that the CMYF board of directors determined did not outweigh the benefits to CMYF and its shareholders expected to be generated by the merger:

The above discussion of the information and factors considered by the CMYF board of directors is not intended to be exhaustive, but includes the material factors the board of directors considered. In reaching its

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determination to approve the merger agreement and recommend that CMYF’s shareholders vote in favor of the merger agreement, the board of directors did not assign any relative or specific weights to the foregoing factors, and individual directors may have given differing weights to different factors. The CMYF board of directors also did not undertake to make any specific determination as to whether any factor was decisive in reaching its ultimate determination. The CMYF board of directors instead based its recommendation on the totality of the information presented.

CMYF’s board of directors unanimously approved the merger agreement and recommends that CMYF’s shareholders vote “FOR” approval of the merger agreement and “FOR” approval of the CMYF adjournment proposal. CMYF shareholders should be aware that CMYF’s directors and executive officers have interests in the merger that are different from, or in addition to, those of other CMYF shareholders. The CMYF board of directors was aware of and considered these interests, among other matters, in evaluating and negotiating the merger agreement, and in recommending that the shareholders of CMYF approve the merger agreement. See “—Interests of Certain Persons in the Merger that are Different than Yours.”

This summary of the reasoning of the CMYF board of directors and other information presented in this section is forward-looking in nature and, therefore, should be read in light of the factors discussed under “Cautionary Statement About Forward-Looking Statements.”

D.A. Davidson’s Opinion to CMYF’s Board of Directors

On March 19, 2018, CMYF entered into an engagement agreement with D.A. Davidson to render financial advisory and investment banking services to CMYF. As part of its engagement, D.A. Davidson agreed to assist CMYF in analyzing, structuring, negotiating and, if appropriate, effecting a transaction between CMYF and another financial institution. D.A. Davidson also agreed to provide CMYF’s board of directors with an opinion as to the fairness, from a financial point of view, of the exchange ratio in the proposed merger to the holders of CMYF’s common stock. CMYF engaged D.A. Davidson because D.A. Davidson is a nationally recognized investment banking firm with substantial experience in transactions similar to the merger and is familiar with CMYF and its business. As part of its investment banking business, D.A. Davidson is continually engaged in the valuation of financial institutions and their securities in connection with mergers and acquisitions and other corporate transactions.

On October 11, 2018, the CMYF board of directors held a meeting to evaluate the proposed merger. At this meeting, D.A. Davidson reviewed the financial aspects of the proposed merger and rendered an opinion to the CMYF board that, as such date and based upon and subject to assumptions made, procedures followed, matters considered and limitations on the review undertaken, the exchange ratio in the proposed merger was fair, from a financial point of view, to the holders of CMYF’s common stock.

The full text of D.A. Davidson’s written opinion, dated October 11, 2018, is attached as Annex C to this proxy statement-prospectus and is incorporated herein by reference. The description of the opinion set forth herein is qualified in its entirety by reference to the full text of such opinion. CMYF’s shareholders are urged to read the opinion in its entirety.

D.A. Davidson’s opinion speaks only as of the date of the opinion and D.A. Davidson undertakes no obligation to revise or update its opinion. The opinion is directed to the CMYF board of directors and addresses only the fairness, from a financial point of view, of the exchange ratio in the proposed merger to the holders of CMYF’s common stock. The opinion does not address, and D.A. Davidson expresses no view or opinion with respect to, (1) the underlying business decision of CMYF to engage in the merger, (2) the relative merits or effect of the merger as compared to any alternative business transactions or strategies that may be or may have been available to or contemplated by CMYF or CMYF’s board of directors, or (3) any legal, regulatory, accounting, tax or similar matters relating to CMYF, its shareholders or relating to or arising out of the merger. The opinion expresses no view or opinion as to any terms or other aspects of the merger, except for the exchange ratio. CMYF and First Interstate determined the exchange ratio through the negotiation process. The opinion does not

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express any view as to the amount or nature of the compensation to any of CMYF’s or First Interstate’s officers, directors or employees, or any class of such persons, or with respect to the fairness of any such compensation. The opinion has been reviewed and approved by D.A. Davidson’s Fairness Opinion Committee in conformity with its policies and procedures established under the requirements of Rule 5150 of the Financial Industry Regulatory Authority.

D.A. Davidson has reviewed the registration statement on Form S-4 of which this proxy statement-prospectus is a part and consented to the inclusion of its opinion to the CMYF board of directors as Annex C to this proxy statement-prospectus and to the references to D.A. Davidson and its opinion contained herein. A copy of the consent of D.A. Davidson is attached as Exhibit 99.2 to the registration statement on Form S-4.

In connection with rendering its opinion, D.A. Davidson reviewed, among other things, the following:

In arriving at its opinion, D.A. Davidson assumed and relied upon the accuracy and completeness of all information that was publicly available, supplied or otherwise made available to, discussed with or reviewed by or for D.A. Davidson. D.A. Davidson relied on the assurances of the managements of CMYF and First Interstate that they are not aware of any facts or circumstances that would make any of such information, forecasts or analyses inaccurate or misleading. D.A. Davidson did not independently verify, and did not assume responsibility for independently verifying, such information or undertaken an independent evaluation or appraisal of any of the assets or liabilities (contingent or otherwise) of CMYF or First Interstate. In addition, D.A. Davidson did not assume any obligation to conduct, nor did D.A. Davidson conduct any physical inspection of the properties or facilities of CMYF or First Interstate and has not been provided with any reports of such physical inspections.

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D.A. Davidson assumed that there has been no material change in CMYF’s or First Interstate’s business, assets, financial condition, results of operations, cash flows, or prospects since the date of the most recent financial statements provided to D.A. Davidson.

With respect to the financial projections and other estimates (including information relating to certain pro forma financial effects of, and strategic implications and operational benefits anticipated to result from, the merger) provided to or otherwise reviewed by or for or discussed with us, we have been advised by management of CMYF that such forecasts and other analyses were reasonably prepared on bases reflecting the best currently available estimates and good faith judgments of management of CMYF as to the future financial performance of CMYF and the other matters covered thereby, and that the financial results (including the potential strategic implications and operational benefits anticipated to result from the merger) reflected in such forecasts and analyses will be realized in the amounts and at the times projected. We assume no responsibility for and express no opinion as to these forecasts and analyses or the assumptions on which they were based.

D.A. Davidson did not make an independent evaluation or appraisal of the loan and lease portfolios, classified loans, other real estate owned or any other specific assets, nor has D.A. Davidson assessed the adequacy of the allowance for loan losses of CMYF or First Interstate. D.A. Davidson has not reviewed any individual credit files relating to CMYF or First Interstate. D.A. Davidson assumed that the respective allowances for loan losses for both CMYF and First Interstate are adequate to cover such losses and will be adequate on a pro forma basis for the combined entity. D.A. Davidson did not make an independent evaluation of the quality of CMYF’s or First Interstate’s deposit base, nor have we independently evaluated potential deposit concentrations or the deposit composition of CMYF or First Interstate. D.A. Davidson did not make an independent evaluation of the quality of CMYF’s or First Interstate’s investment securities portfolio, nor has it we independently evaluated potential concentrations in the investment securities portfolio of CMYF or First Interstate.

D.A. Davidson assumed that all representations and warranties contained in the merger agreement and all related agreements are true and correct in all respects material to D.A. Davidson’s analysis, and that the merger will be consummated in accordance with the terms of the merger agreement, without waiver, modification, or amendment of any term, condition or covenant thereof the effect of which would be in any respect material to D.A. Davidson’s analysis. D.A. Davidson has assumed that all governmental, regulatory or other consents, approvals, and waivers necessary for the consummation of the merger will be obtained without any material adverse effect on the combined company or the contemplated benefits of the merger.

D.A. Davidson assumed in all respects material to its analysis that CMYF and First Interstate will remain as going concerns for all periods relevant to its analysis. D.A. Davidson’s opinion was necessarily based upon information available to D.A. Davidson and economic, market, financial and other conditions as they exist and can be evaluated on the date the fairness opinion letter was delivered to CMYF’s board of directors.

Our opinion does not take into account individual circumstances of specific holders with respect to control, voting or other rights that may distinguish such holders.

We also express no opinion as to the actual value of First Interstate Class A common stock when issued in the merger or the prices at which CMYF’s common stock or First Interstate’s Class A common stock will trade following announcement of the proposed merger or at any future time.

We have not evaluated the solvency or fair value of CMYF or First Interstate under any state, federal or other laws relating to bankruptcy, insolvency or similar matters. This opinion is not a solvency opinion and does not in any way address the solvency or financial condition of CMYF or First Interstate. We are not expressing any opinion as to the impact of the merger on the solvency or viability of CMYF or First Interstate or the ability of CMYF or First Interstate to pay their respective obligations when they come due.

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Set forth below is a summary of the material financial analyses performed by D.A. Davidson in connection with rendering its opinion. The summary of the analyses of D.A. Davidson set forth below is not a complete description of the analysis underlying its opinion, and the order in which these analyses are described below is not indicative of any relative weight or importance given to those analyses by D.A. Davidson. The following summaries of financial analyses include information presented in tabular format. You should read these tables together with the full text of the summary financial analyses, as the tables alone are not a complete description of the analyses.

Unless otherwise indicated, the following quantitative information, to the extent it is based on market data, is based on market data as of October 9, 2018, and is not necessarily indicative of market conditions after such date.

Implied Valuation Multiples for CMYF based on the Merger Consideration

D.A. Davidson reviewed the financial terms of the proposed merger. As described in the merger agreement, each outstanding share of common stock of CMYF will be converted into the right to receive 0.3784 shares of First Interstate Class A common stock, which we refer to as the “exchange ratio.” The terms and conditions of the merger are more fully described in the merger agreement. For purposes of the financial analyses described below, based on the closing price of First Interstate Class A common stock on October 9, 2018 of $45.99, the exchange ratio represented an implied value of $17.40 per share of CMYF common stock. Based upon financial information as of or for the twelve month period ended June 30, 2018 and other financial and market information described below, D.A. Davidson calculated the following transaction ratios:

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Stock Price Performance of First Interstate and CMYF

D.A. Davidson reviewed the history of the reported trading prices and volume of CMYF common stock and First Interstate Class A common stock and certain stock indices, including the Russell 3000 and the KBW Nasdaq Regional Bank Index. D.A. Davidson compared the stock price performance of CMYF or First Interstate with the performance of the Russell 3000 and the KBW Nasdaq Regional Banking Index as follows:

Contribution Analysis

D.A. Davidson analyzed the relative contribution of CMYF and First Interstate to certain financial and operating metrics for the pro forma combined company. Such financial and operating metrics included: (1) market capitalization; (2) net income available for common shareholders during the twelve months ended June 30, 2018; (3) estimates for First Interstate GAAP net income in 2018 and 2019 based on publicly available consensus earnings estimates and estimates for CMYF GAAP net income in 2018 and 2019 based on CMYF management’s budget; (4) total assets; (5) gross loans; (6) total deposits; (7) non-interest bearing demand deposits; (8) non-maturity deposits; and (9) tangible common equity. The relative contribution analysis did not give effect to the impact of any synergies as a result of the proposed merger. The results of this analysis are summarized in the table below, which also compares the results of this analysis with the implied pro forma ownership percentages of CMYF or First Interstate shareholders in the combined company based on the exchange ratio:

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Note: Pro forma contribution does not include any purchase accounting or merger adjustments

Note: Reflects impact from acquisitions closed after June 30, 2018

CMYF Comparable Companies Analysis

D.A. Davidson used publicly available information to compare selected financial and market trading information for CMYF and a group of 14 financial institutions selected by D.A. Davidson that: (1) were headquartered in Idaho, Washington and Oregon; (2) had their common stock listed on an over-the-counter exchange; (3) had assets below $500 million; and (4) were not pending merger targets or ethnic banks. The 14 financial institutions were as follows:

Note: Does not reflect impact from pending acquisitions or acquisitions closed after June 30, 2018

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The analysis compared the financial condition and market performance of CMYF and the 14 financial institutions identified above based on publicly available financial and market trading information for CMYF and the 14 financial institutions as of and for the twelve-month or three-month period ended June 30, 2018. The table below shows the results of this analysis (excluding the impact of earnings per share multiples considered not meaningful by D.A. Davidson).

First Interstate Comparable Companies Analysis

D.A. Davidson used publicly available information to compare selected financial and market trading information for First Interstate and a group of 10 financial institutions selected by D.A. Davidson that: (1) were headquartered in the Western U.S.; (2) had their common stock listed on the NASDAQ or NYSE; (3) had assets between $7.5 billion and $25.0 billion; and (4) were not pending merger targets or ethnic banks. These 10 financial institutions were as follows:

The analysis compared the financial condition and market performance of First Interstate and the 10 financial institutions identified above based on publicly available financial and market trading information for First Interstate and the 10 financial institutions as of and for the twelve-month or three-month period ended June 30, 2018. The analysis also compared the 2018 and 2019 earnings per share multiples for First Interstate and the 10 financial institutions identified above based on publicly available consensus Street estimates for First

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Interstate and the 10 financial institutions. The table below shows the results of this analysis (excluding the impact of earnings per share multiples considered not meaningful by D.A. Davidson).

Precedent Transactions Analysis

D.A. Davidson reviewed three sets of comparable merger and acquisition transactions. The sets of mergers and acquisitions included: (1) “Pacific Northwest Transactions,” (2) “Nationwide Transactions,” and (3) “Nationwide Stock Transactions.”

“Pacific Northwest Transactions” included seven transactions where: