Reddick joined ABC's hit series Lost in 2008. Reddick plays Matthew Abaddon, an attorney for Oceanic Airlines, in multiple episodes. He was the third of four actors from Oz to star in the drama (along with Harold Perrineau, Ken Leung and Adewale Akinnuoye-Agbaje). Producers Damon Lindelof and Carlton Cuse have said they were interested in Reddick for the part of Mr. Eko but he was unavailable due to filming The Wire so they approached Akinnuoye-Agbaje instead.
In early 2008 he was cast in a key role for the pilot of Fringe in which fellow Oz actor Kirk Acevedo also had a regular role. Reddick plays Phillip Broyles, the head of an FBI department investigating paranormal activities. Reddick described the hard-driving character as "a real ass. But he's also one of the good guys." Like Lost, Fringe is co-created and produced by J. J. Abrams.