He began taking lessons before he was in high school and won bit parts in a 1974 "ABC Afterschool Special" entitled "Sara's Summer of the Swans" and the film "Two Minute Warning" (1976). He attended the University of North Carolina at Chapel Hill for two years before transferring to the prestigious Juilliard School. While still studying, he was cast by producer David Puttnam as Virgil, a crew member aboard "Memphis Belle" (1990), a fictional version of the B-17's bombing raid over Germany during WWII.
Even before graduating from Juilliard, Diamond had been cast in a supporting role to Jessica Lange in the CBS/Hallmark Hall of Fame presentation of Willa Cather's "O Pioneers!" (1990). More TV work followed, including a recurring role on "Class of '96" (Fox, 1993) and key roles in "Ironclads" (TNT, 1991), in which he played a Union Spy reporting on the Confederate Merrimack proto-submarine, and another Hallmark presentation, "Blind Spot" (CBS, 1993). He guest starred in an episode of "Law & Order" (NBC), playing a disbarred attorney who schemes with his girlfriend to blackmail her boss. In the fall of 1995, Diamond joined the cast of NBC's "Homicide: Life on the Street" as Det. Mike Kellerman, a veteran of the arson division tested by his reassignment to homicide. What Kellerman lacked in on-hands knowledge and experience, he made up in earnestness and enthusiasm, although he did not have a boy scout past. That same year, Diamond was seen in the Sylvester Stallone feature "Assassins", as well as in three TV movies: "Awake to Danger" and "Her Hidden Truth" (both NBC) and "Secrets" (ABC).