The Better Off Ted star will appear opposite Cuba Gooding Jr in the legal drama, according to TVLine.
Guilty will star Gooding Jr as Billy, a morally ambiguous attorney who is stripped of his licence after being framed.
Harrington will play Scott, a former colleague of Billy who lets his old friend use his new practice as a base of operations.
In addition to playing the lead role in ABC comedy Better Off Ted, the actor has also appeared on Private Practice and Desperate Housewives.
Californication's Amurri Martino will further star in Guilty as another lawyer named Lily.
This Means War director McG will helm the pilot, which has been written by FlashForward's Marc Guggenheim. The pair will also executive produce alongside Peter Johnson and Greg Berlanti (Green Lantern).
Other drama pilots picked up by Fox include an untitled serial killer project with Kevin Bacon and James Purefoy, a teenage spy script from Prison Break's Karyn Usher and a medical drama starring Dexter's Jordana Spiro.
> Read Digital Spy's 2010 interview with Jay Harrington