Defined Contribution Plan	9 Months Ended
Sep. 30, 2023
Retirement Benefits [Abstract]
Defined Contribution Plan	20. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of September 30, 2023, and December 31, 2022, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch-up contribution allowed by law. The Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 5.0% of the participant’s compensation. Employer contributions under the 401(k) Savings Plan were $2.0 million and $5.9 million for the three and nine months ended September 30, 2023, respectively, and $1.2 million and $4.6 million for the three and nine months ended September 30, 2022, respectively, and are included within selling, general, and administrative expense on the unaudited condensed consolidated statements of operations.

Defined Contribution Plan

9 Months Ended

Sep. 30, 2023

20. DEFINED CONTRIBUTION PLAN

On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of September 30, 2023, and December 31, 2022, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch-up contribution allowed by law. The Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 5.0% of the participant’s compensation. Employer contributions under the 401(k) Savings Plan were $2.0 million and $5.9 million for the three and nine months ended September 30, 2023, respectively, and $1.2 million and $4.6 million for the three and nine months ended September 30, 2022, respectively, and are included within selling, general, and administrative expense on the unaudited condensed consolidated statements of operations.