Appeared in Blackboard Jungle (1955) uncredited.
Appeared in four series created by Susan Harris: "Soap" (1977), "The Golden Girls" (1985), "Empty Nest" (1988), and "Nurses" (1991). In the latter three, he played the same character, Dr. Harry Weston.
Ex-wife Rachel Ryan was 29 years younger than he was.
He has a son, James Mulligan.
Mulligan was cremated, and no memorial service was held, at his own request.
Studied playwrighting at Columbia University.
The character "Nagilum", from "Star Trek: The Next Generation" (1987)'s season two episode, "Star Trek: The Next Generation: Where Silence Has Lease (#2.2)" (1988), was named after him for being the first choice to play the role. But the role was given to actor Earl Boen.
Was Jason Robards' stand-by in the 1962 Broadway production of "A Thousand Clowns.".
Won two Emmys for Outstanding Actor in a Comedy Series. However, he finally accepted his Emmy in 1989 for "Empty Nest" (1988) because in 1980 he had been participating in the Screen Actors' Guild strike at the time of the The 32nd Annual Primetime Emmy Awards (1980) (TV), during which he had won for "Soap" (1977).
Worked with both Lee Grant and her daughter, Dinah Manoff. Grant and Mulligan appeared in the movie Teachers (1984); Mulligan and Manoff appeared in the sitcoms "Soap" (1977) and "Empty Nest" (1988).
Younger brother of director Robert Mulligan.