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Mandiant rose to prominence last year after it alleged that a secretive branch of China's military had stolen data from more than 100 global firms.
The deal, one of the largest ones in the sector recently, comes amid increased worries over cyber security.
FireEye shares rose 24% in after hours trading in New York on the deal.
The companies said they had agreed the deal on 30 December, but only made it public on Thursday after close of US markets.
FireEye and Mandiant had entered into a technology development agreement in 2013.
"Organizations today are faced with knitting together a patchwork of point products and services to protect their assets from advanced threats," David DeWalt, chief executive of FireEye, said in a statement.
"Together, the size and global reach of FireEye and Mandiant will enable us to innovate faster, create a more comprehensive solution, and deliver it to organizations around the world at a pace that is unmatched by other security vendors."
Kevin Mandia, Mandiant's founder and chief executive officer prior to the acquisition, has been appointed as the chief operating officer of FireEye.