First Interstate BancSystem (FIBK) 8-K First Interstate BancSystem, Inc. Completes

Exhibit 99.1

First Interstate BancSystem, Inc. Completes

Acquisition of Community 1st Bank

Company Release – April 8, 2019

(BILLINGS, Mont) First Interstate BancSystem, Inc. (NASDAQ: FIBK)(“First Interstate”), parent company of First Interstate Bank, announced that, effective today, it completed its acquisition of Community 1st Bank (“CMYF”). Immediately following the acquisition, CMYF was merged into First Interstate Bank, with First Interstate Bank as the resulting institution.

“Community 1^st Bank has a long-standing history of service excellence and giving back to the community—a tradition we also champion at First Interstate Bank,” said Kevin Riley, First Interstate President and Chief Executive Officer. “In welcoming Community 1^st to the fold, we are not only expanding upon our presence in some of Idaho’s most robust markets, we are building upon a legacy and service philosophy of ‘doing the right thing.’ Culturally, this should allow for a seamless integration of our institutions.”

Following the merger, CMYF clients will continue to enjoy a highly-personalized banking experience. First Interstate offers an expanded line of financial services and innovative solutions, such as Wealth Management, Mortgage, Treasury Solutions, and a unique credit card with a rewards program featuring travel benefits and perks from local merchants.

In accordance with the definitive agreement, approximately 500,000 shares of First Interstate Class A common stock were issued. Piper Jaffray & Co. served as financial advisor, and Luse Gorman, PC, served as legal counsel to First Interstate. D.A. Davidson & Co. served as financial advisor, and Breyer & Associates PC served as legal counsel to CMYF.

About First Interstate BancSystem, Inc.

First Interstate BancSystem, Inc. is a financial and bank holding company incorporated in 1971 and headquartered in Billings, Montana. First Interstate operates banking offices, including detacheddrive-up facilities, in communities across Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. First Interstate delivers a comprehensive range of banking products and services to individuals, businesses, municipalities, and other entities throughout First Interstate’s market areas.

Cautionary Note Regarding Forward Looking Statements

This press release contains “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, and Rule 175 promulgated thereunder, and Section 21E of the Securities Exchange Act of 1934, as amended, and Rule3b-6 promulgated thereunder, that involve inherent risks and uncertainties. These forward-looking statements include, but are not limited to, statements about (i) the benefits of the merger between First Interstate and CMYF, including anticipated future results, cost savings and accretion to reported earnings that may be realized from the merger; (ii) First Interstate and CMYF’s plans, objectives, expectations and intentions and other statements contained in this presentation that are not historical facts; and (iii) other statements identified by words such as “expects,” “anticipates,” “intends,” “plans,” “believes,” “seeks,” “estimates” or words of similar meaning. Annualized, pro forma, projected and estimated numbers are used for illustrative purpose only, are not forecasts and may not reflect actual results.

Forward-looking statements involve risks and uncertainties that may cause actual results to differ materially from those in such statements. The following factors, among others, could cause actual results to differ materially from the anticipated results expressed in the forward-looking statements: the cost savings from the merger may not be fully realized or may take longer than expected to be realized; operating costs, customer loss and business disruption following the merger may be greater than expected; the interest rate environment may further compress margins and adversely affect net interest income; the risks associated with continued diversification of assets and adverse changes to credit quality; and difficulties associated with achieving expected future financial results. Additional factors that could cause actual results to differ materially from those expressed in the forward-looking statements are discussed in First Interstate’s reports (such as the Annual Report on Form10-K, Quarterly Reports on Form10-Q and Current Reports on Form8-K) filed with the Securities and Exchange Commission (the “SEC”) and available at the SEC’s Internet website (www.sec.gov). All subsequent written and oral forward-looking statements concerning the proposed transaction or other matters attributable to First Interstate or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements above. Except as required by law, First Interstate does not undertake any obligation to update any forward-looking statement to reflect circumstances or events that occur after the date the forward-looking statement is made.

For further information:

First Interstate BancSystem, Inc.

Marcy Mutch, Chief Financial Officer

406-255-5312

marcy.mutch@fib.com

or

MEDIA:

Brittany Cremer, PR/Communications Manager

406-255-5310

brittany.cremer@fib.com