| Cover | Oct. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 4, 2021 |
| Entity Registrant Name | Mandiant, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36067 |
| Entity Tax Identification Number | 20-1548921 |
| Entity Address, Address Line One | 601 McCarthy Blvd. |
| Entity Address, City or Town | Milpitas |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95035 |
| City Area Code | 408 |
| Local Phone Number | 321-6300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Trading Symbol | MNDT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001370880 |
| Amendment Flag | false |
