Sloyan has also appeared in a number of science fiction television series, including Buck Rogers in the 25th Century, in which he portrayed Barnard "Barney" Smith in the episode "The Plot to Kill a City", as well as several roles in the Star Trek franchise. In Star Trek: The Next Generation, he portrayed Alidar Jarok (a defecting Romulan admiral) in "The Defector", and Alexander Rozhenko (Worf's son) as an adult in the Future, in "Firstborn". In Star Trek: Deep Space Nine, he portrayed the Bajoran scientist Mora Pol and Odo's "father" in the episodes "The Begotten" and "The Alternate". The Star Trek: Voyager episode entitled "Jetrel" featured Sloyan as the title character.

Sloyan is featured in the opening sequence of The Sting. "Mottola", Sloyan's character, was used to illustrate the concept of a "griftee" in the film, in a variation on the pigeon drop scam, having been "cleaned by two grifters" who "...tied into [Sloyan's Character] on 47th...played the switch for him, then blew him off to a cab on 49th." Sloyan's character in the movie was eventually murdered by the film's antagonist.

Sloyan left the United States at an early age to live abroad in Rome, Capri, Milan, Switzerland, and Ireland. Sloyan's career in show-business began upon his return to the United States, in 1957, where Sloyan received a scholarship to the American Academy of Dramatic Arts. Sloyan's acting career was interrupted in 1962, when he was drafted into the United States Army during its operations in Vietnam.

He has also played roles in The Traveling Executioner (1970), The Gang That Couldn't Shoot Straight (1971), and Xanadu (1980).

Sloyan and wife, Deirdre Lenihan, an Actress, have been married since 1973. They are the parents of actor Dan Sloyan and Actress Samantha Sloyan.

Sloyan was also "the voice of Lexus", having performed voice-overs in American television advertisements for Lexus since the make's introduction to the American market. In 2009, he was replaced by actor James Remar. Sloyan now voices ads for Mitsubishi.