He had a recurring role on the first two seasons of Designated Survivor as John Foerstel, Director of the FBI, and portrayed Daniel Whitehall / Werner Reinhardt in the Marvel Cinematic Universe (MCU) television series Agents of S.H.I.E.L.D.

[2] Diamond attended the University of North Carolina at Chapel Hill for two years,[2] then studied acting at the Juilliard School, where he was a member of the Drama Division's Group 20 (1987–1991).

[3] His early roles included guest starring on Law & Order and Class of '96, and such films as Memphis Belle (1990), Clear and Present Danger (1994) and Her Hidden Truth (1995).

He was credited as one of the main cast members and featured prominently in promotional pictures of the show, so as to surprise viewers when he was murdered at the end of the first episode.

In the 2000s, as well as playing recurring roles in Judging Amy (as Stuart Collins) and Dollhouse (as Lawrence Dominic), Diamond guest-starred on Crossing Jordan, The West Wing, Medium, Numb3rs, CSI: Crime Scene Investigation, Stargate SG-1, Without a Trace, The Mentalist, 24, and Common Law.