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.