Montrose Environmental Group, 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

Filed by the registrant☒ Filed by a party other than the registrant☐

Check the appropriate box:

Montrose Environmental Group, Inc.

(Name of registrant as specified in its charter)

N/A

(Name of person(s) filing proxy statement, if other than the registrant)

Payment of filing fee (Check all boxes that apply):

To Our Stockholders:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders of Montrose Environmental Group, Inc. The Annual Meeting will be held virtually on Wednesday, May 6, 2026, at 11:30 AM, 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 of record and verified beneficial holders 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 2026 Annual Meeting of Stockholders and Proxy Statement which follow this letter. The Company’s 2025 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 2025 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 2025 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 the e-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 on 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.

Sincerely,

Vijay Manthripragada

President and

Chief Executive Officer

2026 Notice of Annual Stockholders Meeting and Proxy Statement