As a child, Diamond had small parts in the ABC 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).
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).
Diamond was married to Fredrika Kesten from 1995 to 1997. Since 2004, he has been married to Actress Marnie McPhail.
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, CSI: Crime Scene Investigation, Stargate SG-1, Without a Trace, The Mentalist, 24, and Common Law. He portrayed Inspector Jack Vasser in the Fall 2007 NBC 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 & Bash. 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, Dr. Ryan's fiancee in Common Law on the USA network. He is currently playing senior HYDRA agent Dr. Daniel Whitehall in Marvel's Agents of S.H.I.E.L.D. and played toymaker Harold Ballinger in the mystery series Wayward Pines.
