Shares of the company, which lowered its full-year revenue forecast, fell 8.4 percent to $14.64 in after-hours trading on Thursday.
Mandia currently leads the company's Mandiant forensics unit, which has probed some of the biggest cyber-attacks to date, including the 2013 holiday attack on Target Corp and the attack on Sony Pictures Entertainment.
Mandiant was founded in 2004 by Mandia, a former US Air Force cyber-forensics investigator who has also co-authored an influential textbook on the subject.
FireEye bought Mandiant in a cash-and-stock deal worth $1.05 billion in January 2014, within months of going public in September 2013.
DeWalt, who is also FireEye's chairman, will stay on as executive chairman, the company said. He has been chief executive since November 2012.
The company also named former Symantec Corp Chief Executive Enrique Salem as lead independent director.
FireEye said it now expects full-year total revenue in the range of $780 million-$810 million, lower than its previous forecast of $815 million-$845 million.
Analysts on average had expected revenue of $828.6 million.
The Milpitas, California-based company is facing rising competition from a number of vendors such as Palo Alto Networks Inc , Proofpoint Inc and Imperva Inc for a larger share of the market, which is showing signs of slowing growth.
The company cut about 200 jobs in the first quarter and through the current quarter, Chief Financial Officer Michael Berry said on a post-earnings conference call.
FireEye had about 3,100 employees as of Dec. 31.
The company, which provides Web, email and malware security to businesses and governments, said its revenue jumped 34 percent to $168 million in the first quarter ended March 31, from $125.4 million a year earlier.
However, the net loss attributable to shareholders widened to $155.9 million, or 98 cents per share, from $134 million, or 88 cents per share.
Excluding items, the company lost 47 cents per share.
Analysts on average had expected revenue of $171.8 million and a loss of 50 cents, according to Thomson Reuters I/B/E/S.
Up to Thursday's close, shares had fallen 23 percent this year, underperforming the 5.8 percent fall in the broader Nasdaq Composite index.
© Thomson Reuters 2016