Helfer will play senior partner Alex Clark of the firm Kinross & Clark on the show, according to TV Line, and will join Josh Lucas, Juliette Lewis, Molly Parker and The Killing's Callum Keith Rennie.
Based on John Grisham's best-selling novel, the forthcoming NBC series is set ten years after the events of the book, with protagonist Mitch McDeere (Lucas) emerging from the Witness Protection programme to confront old threats.
The actress tweeted the news yesterday, saying that she is thrilled to be working on the lawyer drama.
"[I'll] be working with some amazing talent," Helfer wrote. "[I'm] very excited!"
The Firm was previously made into a movie starring Tom Cruise and Jeanne Tripplehorn in 1993.
The series is scheduled to begin production in August for a 2012 debut.
Watch the trailer for 1993's The Firm below: