His film roles include Galen in Dragonslayer (1981), Stingo in Sophie's Choice (1982), Janosz Poha in Ghostbusters II (1989), Gary Granger in Addams Family Values (1993), Renfield in Dracula: Dead and Loving It (1995), and David Langley in Bean (1997).

MacNicol won the Primetime Emmy Award for Outstanding Supporting Actor in a Comedy Series in 2001 for his role as the eccentric lawyer John Cage in the FOX comedy-drama Ally McBeal (1997–2002).

MacNicol was born on April 10, 1954, in Dallas, Texas, the youngest of five children of Barbara Jean (née Gottlich), a homemaker, and John Wilbur Johnson, a Norwegian-American corporate executive who became an Episcopal priest later in life.

[8] In 1987, MacNicol starred in the Trinity Repertory Company's original production of the stage adaptation of Robert Penn Warren's All the King's Men, which first appeared at the Dallas Theater Center.

He has appeared in repertory theater, including the New York Shakespeare Festival where he played title roles in Richard II and Romeo and Juliet; and in Twelfth Night, Rum and Coke and Found a Peanut.

[10][8] In film, he plays the naive Southern writer who falls 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.

From 1992 to 1993 MacNicol starred opposite John Forsythe, Holland Taylor, David Hyde Pierce and Joseph Gordon-Levitt as press secretary Bradley Grist in the political television comedy The Powers That Be.