Rosenberg moved to Los Angeles in 1983 and landed his first TV job in the miniseries "Robert Kennedy and His Times" (1985) playing newspaper columnist Jack Newfield. After this breakthrough, he began working steadily turning up in supporting roles in such TV-movies as "Kojak: The Belarus File" (1985), as gangster Frank Nitti in "The Revenge of Al Capone" (1989) and as homicide victim Jennifer Levin's father in "The Preppie Murder" (1989). The actor was featured alongside his real-life second wife Marg Helgenberger in the 1991 PBS presentation "Peacemaker". (They subsequently acted together in the 1994 Peter Weller-directed short "Partners" which aired on Showtime and the 1998 Lifetime TV-movie "Giving Up the Ghost".

Following an Emmy-nominated 1994 guest turn on NBC's "ER", he moved to sitcom land when he was cast as Cybill Shepherd's divorced second husband Ira Woodbine on "Cybill" (CBS, 1995-98), a mid-season series replacement about an underemployed soap actress, her kooky best friend, her daughters and her two ex-husbands. When that series ended, Rosenberg next joined the cast of "Chicago Hope" in the role of lawyer Stuart Brickman. Impressing the producers, the role was upped from recurring to regular status. Rosenberg then played the meticulous head of a child advocacy office in the CBS drama "The Guardian" (2001- ). After a string of guest-starring roles on television--including a 2005 stint opposite Helgenberger on her hit series "CSI" Crime Scene Investigation"--and a voice role as Jack Hammer in the CGI animated feature robots, Rosenberg was elected president of the Screen Actors Guild in September 2005 after campaigning to push the actors' union into more aggressive negotiations with studios over such issues as DVD sales residuals and end the in-fighting that has long plagued the group.