Onterris, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 14A

INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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2024 Notice of Annual Stockholders Meeting & Proxy Statement

DATE & TIME: May 7, 2024, 9:00 a.m. CT LOCATION: Online via live webcast at www.proxydocs.com/MEG

To Our Stockholders:

You are cordially invited to attend the 2024 Annual Meeting of Stockholders of Montrose Environmental Group, Inc. The Annual Meeting will be held virtually on Tuesday, May 7, 2024, at 9:00 a.m., Central time, over the internet. Stockholders can attend and participate in, including submitting questions and examining our stockholder list, the Annual Meeting by visitingwww.proxydocs.com/MEG, the meeting website, and entering the requested information such as a valid control number. This meeting website will also include links for stockholders to vote their shares during the meeting. See “Q&A About Our Annual Meeting—How Do I Attend the Annual Meeting?” and “Q&A About Our Annual Meeting—How Do I Vote at the Annual Meeting?” in the Proxy Statement for more details.

We describe in detail the actions we expect to take at our Annual Meeting in the Notice of 2024 Annual Meeting of Stockholders and Proxy Statement which follow this letter. The Company’s 2023 Annual Report on Form 10-K is also being made available to stockholders along with these materials.

We have mailed to many of our stockholders a Notice of Internet Availability of Proxy Materials containing instructions on how to access our Proxy Statement and our 2023 Annual Report to Stockholders, and how to vote on the proposals put before you this year. This Notice of Availability also includes instructions on how to request a paper or email copy of the proxy materials, including the Notice of Annual Meeting, Proxy Statement and 2023 Annual Report, and proxy card or voting instruction form. Stockholders who previously either requested paper copies of the proxy materials or elected to receive the proxy materials electronically did not receive a Notice of Availability and will receive the proxy materials in the format requested. In addition, by following thee-consent instructions in the proxy card or voting instruction form, stockholders may choose to go paperless in future solicitations and request proxy materials electronically by email on an ongoing basis.

Please use this opportunity to take part in the affairs of our company by voting on the business to come before the Annual Meeting. Whether or not you plan to attend the Annual Meeting, please complete, sign, date, and return the accompanying proxy card or voting instruction form or vote electronically on the Internet or by telephone. See “Q&A About Our Annual Meeting—How Do I Vote?” in the Proxy Statement for more details. Returning the proxy card or voting instruction form or voting electronically over the Internet or by telephone does not deprive you of your right to attend the Annual Meeting or to vote your shares during the Annual Meeting up to and through the applicable deadlines set forth in your proxy materials.

Sincerely,

Vijay Manthripragada

President and Chief Executive Officer

2024 Notice of Annual Stockholders Meeting and Proxy Statement

Voting Items

The Annual Meeting of Stockholders (the “Annual Meeting”) of Montrose Environmental Group, Inc., a Delaware corporation (the “Company”), is being held for the stockholders to consider the following proposals:

Stockholders will attend to other matters that may properly come before the Annual Meeting or any adjournment or postponement thereof.

Materials for Review

We encourage you to review the proxy materials for our Annual Meeting, which include our Proxy Statement and our 2023 Annual Report. On March 25, 2024, we made our proxy materials available onwww.proxydocs.com/MEG and began mailing a Notice of Internet Availability of Proxy Materials or, if previously requested, paper or electronic copies of the proxy materials, to our stockholders. Please review the Notice of Availability for information on how to access the proxy materials and how to vote.

Attending the Meeting

Stockholders of record as of the close of business on March 13, 2024, the Record Date, are entitled to attend, participate in and vote at the Annual Meeting. To access and participate in the Annual Meeting, including submitting questions and viewing the list of registered stockholders as of the Record Date, and to vote during the Annual Meeting, stockholders of record should go to the meeting website atwww.proxydocs.com/MEG, enter the control number found on your proxy materials such as your proxy card, your voting instruction form or your Notice of Availability of Proxy Materials, and follow the instructions on the website. Beneficial owners should review these proxy materials and their voting instruction form for how to vote in advance of and how to participate in the Annual Meeting.

In the event of a technical malfunction or other situation that the meeting chair determines may affect the ability of the Annual Meeting to satisfy the requirements for a stockholder meeting to be held by means of remote communication under the Delaware General Corporation Law, or that otherwise makes it advisable to adjourn the Annual Meeting, the chair will convene the Annual Meeting at 9:30 a.m. Central time on the date specified above and at the meeting website address specified above solely for the purpose of adjourning the Annual Meeting to reconvene at a date, time and physical or virtual location announced by the meeting chair. Under either of the foregoing circumstances, we will post information regarding the announcement on the investors page of the Company’s website atwww.montrose-env.com.

We encourage you to review these proxy materials and vote your shares before the Annual Meeting.

FOR THE BOARD OF DIRECTORS

Nasym Afsari

General Counsel and Secretary

5120 Northshore Drive

North Little Rock, AR 72118

March 25, 2024