| Cover | Dec. 31, 2021 | 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 | |
| Entity Information, Former Legal or Registered Name | FireEye, Inc. | |
| 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 | FEYE | |
| Security Exchange Name | NASDAQ | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0001370880 | |
| Amendment Flag | false | |
| Current Fiscal Year End Date | --12-31 |
