MacNicol performed for two seasons from 1978 at the Guthrie Theater in Minneapolis, including in productions of Hamlet and The Pretenders. He made his New York debut in the 1980 off-Broadway play, Crimes of the Heart. The production eventually moved to Broadway in 1981, and he won the Theatre World Award. It was also during this production that a casting agent noticed him and called him in to read for his eventual role in Sophie's Choice. In 1981 he landed the starring role in his first film, Dragonslayer, opposite Sir Ralph Richardson. In 1987, he starred in the Trinity Repertory Company's original production of the stage adaptation of All the King's Men, which first appeared at the Dallas Theater Center. This adaptation was developed with the consultation of the author himself. Among his other stage credits is the Broadway production of Black Comedy/White Lies. He has appeared in repertory theater, including the New York Shakespeare Festival in which he played title roles in Richard II and Romeo and Juliet, and appeared in Twelfth Night, Rum and Coke and Found a Peanut. On film, he played the naive Southern writer who fell in love with Meryl Streep in Sophie's Choice, the museum curator Janosz Poha in Ghostbusters II, and camp director Gary Granger alongside future Numbers co-star David Krumholtz in Addams Family Values. Other film credits include Housesitter and American Blue Note. From 1992-1993 he starred opposite John Forsythe, Holland Taylor, David Hyde Pierce and Joseph Gordon-Levitt as press secretary Bradley Grist in the short-lived political comedy The Powers That Be. In 1994 MacNicol played the role of Alan Birch for the first season and under half of the second season of Chicago Hope once creator David E. Kelley departed. He later rejoined Kelley in 1997 by taking a role on another TV series, Ally McBeal. However, he did return for one final guest appearance in episode five of the former show's fifth season for the 100th episode. MacNicol is known by for his Ally McBeal performance as eccentric attorney John Cage, for which he won an Emmy Award for Outstanding Supporting Actor in a Comedy Series in 2001. He also starred in the drama Numbers as physicist Dr. Larry Fleinhardt, and had a role as Tom Lennox in the sixth season of the hit FOX show 24. MacNicol reprised his role as Lennox in the film '. He also played a hotel receptionist in one episode of Cheers titled "Look Before You Sleep". MacNicol has lent his voice to several comic book supervillains: Dr. Kirk Langstrom/Man-Bat in The Batman, David Clinton/Chronos in Justice League Unlimited, Professor Ivo in Young Justice, Dr. Otto Octavius/Doctor Octopus in The Spectacular Spider-Man, X The Eliminator in Harvey Birdman, Attorney at Law and the Mad Hatter in the video games', ', and '. He also voiced Firefly in . MacNicol played Dr. Stark, a pediatric surgeon, on Grey's Anatomy. MacNicol was nominated for an Emmy for outstanding guest actor in the fifth season of Veep; however, this was rescinded after he appeared for all of ten seconds in a fifth episode of the season's ten episode run, hence disqualifying him as a part-time "guest" and joining him in the ranks of Dennis Miller and Henry Winkler whose nominations were also revoked. He was later nominated in the same category for the seventh season of Veep.
