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Document and Entity Information - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2020
Mar. 19, 2021
Cover [Abstract]    
Document Type 10-K  
Amendment Flag false  
Document Period End Date Dec. 31, 2020  
Document Annual Report true  
Document Transition Report false  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus FY  
Trading Symbol MEG  
Entity Registrant Name Montrose Environmental Group, Inc.  
Entity Central Index Key 0001643615  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   25,200,343
Entity Public Float   $ 1.2
Entity Well-known Seasoned Issuer No  
Entity Voluntary Filers No  
Entity Interactive Data Current Yes  
Entity Current Reporting Status Yes  
ICFR Auditor Attestation Flag false  
Entity Shell Company false  
Entity File Number 001-39394  
Entity Tax Identification Number 46-4195044  
Entity Incorporation State Country Code DE  
Entity Address Address Line1 1 Park Plaza  
Entity Address, Address Line Two Suite 1000  
Entity Address City Or Town Irvine  
Entity Address State Or Province CA  
Entity Address Postal Zip Code 92614  
City Area Code 949  
Local Phone Number 988-3500  
Security12b Title Common Stock, par value $0.000004 per share  
Security Exchange Name NYSE  
Documents Incorporated by Reference

Portions of the registrant’s Proxy Statement for the 2021 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. Such proxy statement will be filed with the Securities and Exchange Commission within 120 days of the registrant’s fiscal year ended December 31, 2020.