Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in Registration Statement No. 333-248533 on Form S-8 of our report dated March 24, 2021, relating to the financial statements of Montrose Environmental Group, Inc. appearing in this Annual Report on Form 10-K for the year ended December 31, 2020.

/s/ Deloitte & Touche LLP

Costa Mesa

March 24, 2021