Background
Diamond was born in Brooklyn, the son of Allison, an astrologer, and Bob Diamond, a stage manager and director of shows such as The Joe Franklin Show.
Diamond was born in Brooklyn, the son of Allison, an astrologer, and Bob Diamond, a stage manager and director of shows such as The Joe Franklin Show.
He was raised in Manhattan and attended the Trinity School. Diamond attended the University of North Carolina at Chapel Hill for two years, then studied acting at the Juilliard School, where he was a member of the Drama Division"s Group 20 (1987–1991).
He is best known for the roles of Detroit Mike Kellerman on Homicide: Life on the Street, Jason Pillar in season 8 of 24, and recurring character Laurence Dominic on Dollhouse. He also appeared in The Shield, Bones, The Mentalist, Franklin & Bash and Agents of South.H.I.E.L.D.
As a child, Diamond had small parts in the American Broadcasting Company Afterschool Special Sara"s Summer of the Swans (1974) and the film Two Minute Warning (1976).
His early roles as an adult 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). His first big break came in 1995 when he joined the cast of Homicide: Life on the Street as Detroit
Mike Kellerman. Diamond was a regular cast member for Seasons 4-6 before Kellerman was written out at the end of Season 6.
But he reprised the role for a two-episode story in the final Season 7. In 2000, a feature-length television movie was produced to wrap up the series, which saw the return of all the major cast, including Diamond.
He made a brief reappearance as a police detective in the pilot episode of the 2002 pilot of the controversial series The Shield, as Detective Terry Crowley. 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.
Recent roles include a re-make of High Noon (2000).
Three Days. South.W.A.T. (2003, directed by Diamond"s Homicide co-star Clark Johnson). Spider-Manitoba 2 (2004); and Good Night, and Good Luck (2005).
As well as playing the recurring role of Stuart Collins on Judging Amy from 1999 to 2003, Diamond has guest starred on Crossing Jordan, The West Wing, Medium, Numb3rs, Computer Society of India: Crime Scene Investigation, Stargate SG-1, Without a Trace, The Mentalist, 24, and Common Law.
He portrayed Inspector Jack Vasser in the Fall 2007 National Broadcasting Company series Journeyman until the cancellation of the show in December 2007. As of spring 2010, Diamond has joined the cast of the TNT series Franklin & Bashkir In 2010, he voiced Barnes in Firebreather.
In 2011, he had a part in Moneyball, as a general manager of the Cleveland Indians.
In 2012 he played Robert Yule, Doctor Ryan"s fiancee in Common Law on the United States of America network. He is currently playing senior HYDRA agent Doctor Daniel Whitehall in Marvel"s Agents of South.H.I.E.L.D. and played toymaker Harold Ballinger in the mystery series Wayward Pines.