Martin Mull -- famous for his work on "Roseanne" -- is dead.

The longtime actor died Thursday at his home, the result of a long illness ... according to his rep, Jennifer Craig.

Martin played Leon Carp on "Roseanne" back in the day, appearing in 46 episodes. He was also a regular on "Sabrina the Teenage Witch" and had a recurring role in "Two and a Half Men."

He broke into TV in 1976 on the comedic soap opera, "Mary Hartman, Mary Hartman," and starred alongside Stephanie Faracy in 1990's "His & Hers."

On the big screen, Martin is most known for playing Colonel Mustard in 1985's "Clue." He also had roles in "Jingle All The Way" and "Mr. Mom" ... among other films.

Martin's daughter, Maggie, says her late father "was known for excelling at every creative discipline imaginable and also for doing Red Roof Inn commercials. He would find that joke funny. He was never not funny.”

In addition to acting, Martin also painted, published a book and made several comedy albums that were nominated for Grammy Awards.

He's survived by his wife, Wendy, and their daughter.

Martin was 80.

RIP