Liam Sullivan (May 18, 1923 – April 18, 1998) was an American actor, singer and comedian. Sullivan was born in Jacksonville, Illinois.
Over his acting career, Sullivan played many roles in multiple television series. He and Angie Dickinson appeared together in the episode "Point of Honor" of the syndicated American Civil War drama Gray Ghost. He guest starred as Jason Douglas in the 1960 episode "The Target" of the syndicated western series Tombstone Territory, with fellow guest star Warren Oates. He appeared as Nexus in the Lost in Space episode "His Majesty Smith," Mr. Willis in Knots Landing, Dr. Burt Hammond in St. Elsewhere, and Mr. Plenn in Falcon Crest and he played the prosecuting attorney in the Hill 256 Episode of Combat!. He also appeared in movies such as That Darn Cat! as Graham and The Magic Sword. He is perhaps best remembered today for his role as the supremely arrogant and cruel telepathic alien, Parmen, in the Star Trek episode "Plato's Stepchildren"; he was also featured in the Twilight Zone episodes The Changing of the Guard and The Silence. Telepathy was also the subject of experiments Sullivan conducted in the documentary film from 1977 called The Amazing World of Psychic Phenomena hosted by Raymond Burr.
Sullivan often portrayed villains throughout his acting career. He once said, "Playing truly evil people is a great way to release tension and anger and disgust with humanity. Show bad people what they really look and act like and maybe they'll recognize themselves and change. Who knows?"
In the 1966-1967 season, Sullivan, at forty-three, appeared as Major Mapoy in all twenty-six episodes of the ABC Western series The Monroes with Michael Anderson, Jr., and Barbara Hershey. He is buried in Jacksonville's Diamond Grove Cemetery.