Diamond was born and raised in New York City. He attended school at UNC-Chapel Hill and then went on to study acting at The Juilliard School in New York. His professional career began in 1990 with his co-starring role in the movie Memphis Belle (1990). Following that, he appeared on Broadway, and in many television movies, and guest-star roles on TV.He is probably best known for playing "Detective Mike Kellerman" on the critically acclaimed NBC television show Homicide: Life on the Street (1993).Diamond most recently guest-starred on television in, "October Road," Ghost Whisperer (2005), Vanished (2006), Without a Trace (2002), Numb3rs (2005), Law & Order (1990), "CSI", Medium (2005), The West Wing (1999), Judging Amy (1999), Crossing Jordan (2001), and The Shield (2002).Some of his most notable movie credits include Good Night, and Good Luck. (2005), Spider-Man 2 (2004) and S.W.A.T. (2003).Diamond resides in Los Angeles with his wife, actress Marnie McPhail.