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).