Moving east, he made his NYC stage debut in the Broadway production of Beth Henley's "Crimes of the Heart" (1981) and went on to perform frequently at Joseph Papp's New York Shakespeare Festival. MacNicol segued into TV with the superior TV-movie, "Johnny Bull" (ABC, 1986). Developed at the National Playwrights Conference of the Eugene O'Neill Theater Company, this drama afforded the younger actor an opportunity to work with old pros Jason Robards and Colleen Dewhurst.
Though MacNicol initially established himself in dramatic fare, he would spend a substantial portion of his film and TV career in comedies, doing guest shots on "The Days and Nights of Molly Dodd" and "Cheers", and landing his first stint as a regular on Norman Lear's political satire "The Powers That Be" (NBC, 1992-93), playing an obnoxious senatorial aide. For one season (1994-95), he played the hospital counsel coping with single fatherhood on "Chicago Hope" (CBS) whose shocking death left many fans of the show upset. MacNichol eventually returned to the weekly grind in 1997 as the quirky law firm head John 'Biscuit' Cage on the hit Fox series "Ally McBeal", created and written by "Chicago Hope" creator David E Kelley. The eccentric lawyer was a perfect match for the performer's comedic gifts and he earned successive Emmy nominations in 1999. 2000 and 2001, (finally taking home the statue in 2001). Additionally, MacNicol moved behind the cameras to helm episodes.
Despite his small screen successes, MacNicol has enjoyed several triumphs in feature character roles that have provided showcases for his quirky persona. He received good notices for his lively portrayal of a bewitched art restorer with an impenetrable accent in the blockbuster sequel "Ghostbusters II" (1989), and turned in gemlike comic supporting roles as a perky camp counselor in "Addams Family Values" (1993), the bug-eating sidekick Renfield in Mel Brooks' "Dracula: Dead and Loving It" (1995), and a museum curator exasperated by his oddball houseguest (Rowan Atkinson) in "Bean" (1997).