SAN FRANCISCO — In a deal that may have broad repercussions for companies and governments fending off sophisticated hackers and state-sponsored digital attacks, FireEye, a provider of security software, has acquired Mandiant, a company known for emergency responses to computer network breaches.

The deal, in both cash and stock, is worth more than $1 billion, based on the current value of shares in FireEye.

The acquisition, which closed on Monday but was not publicly announced until after the markets closed on Thursday, was one of the biggest security deals of 2013. It merges two darlings in the $67 billion global computer security market that together could form a formidable competitor to antivirus giants like Symantec and Intel’s McAfee.

David G. DeWalt, FireEye’s chairman and chief executive, ran McAfee before it was sold to Intel in 2010. Mr. DeWalt was rumored to be a contender for the top job at Intel, but surprised company insiders when he left to join FireEye in 2012.