Mull was born in Chicago, IL, and moved with his family to North Ridgeville, OH, when he was two years old. They stayed there until he was 15 years old, when his family moved to New Canaan, CT, where he attended and graduated from high school. He studied painting and went on to graduate from the Rhode Island School of Design with a Bachelor of Fine Arts and a Master of Fine Arts in painting.

Mull has been a painter since the 1970s, and has had his work appear in group and solo exhibits throughout that time. His work often combines photorealist painting, and the pop art and collage styles. He published a book of some of his paintings, entitled "Paintings Drawings and Words," in 1995. One of his paintings was used on the cover for the 2008 Joyce Carol Oates novel "My Sister, My Love."

Mull's first famous acting role was as twins Garth and Barth Gimble in the 1976 television nighttime absurdist soap opera "Mary Hartman, Mary Hartman." This led to work in the spin-off comedy talk shows "Fernwood 2 Night" (1977) and "America 2-Night" (1978), in which he played Barth Gimble as talk show host, opposite Fred Willard as sidekick Jerry Hubbard.

In one episode of "The Golden Girls," he played a hippie who was afraid of the outside world. In 1979, he appeared in the "Taxi" episode 'Hollywood Calling.' Mull created, wrote for and starred in the short-lived 1984 CBS sitcom "Domestic Life," with Megan Follows playing his teenage daughter. He had a long-running role playing Roseanne Conner's gay boss on the TV series "Roseanne."

During the 1980s, Mull starred in a series of commercials for Michelob and Pizza Hut, and in a series of television and radio commercials for Red Roof Inn. He appeared in the 'Pecos Bill' episode of the Shelley Duvall TV series "Tall Tales & Legends." Mull voiced the role of The Evil Cad on the 1995-97 animated series "Freakazoid!" He also did the voice of Vlad Masters/Vlad Plasmius, the main villain in "Danny Phantom." He also played Vice Principal Willard Kraft on "Sabrina, The Teenage Witch."

Mull has appeared as a guest star on the game show "Hollywood Squares," appearing as the center square in the show's final season, from 2003 to 2004. In late 2004, he portrayed Gene Parmesan, a private investigator, on the TV series "Arrested Development". During 2008 and 2009, Mull guest starred in two episodes of the television series "Gary Unmarried" as Allison's father.

Mull will star in Fox's new TV series, "Dads".

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