Starting with an appearance on TV's "Fame" in 1985, Clemenson began a decades-long résumé of smaller parts in such films as Heartburn, The Big Lebowski and Apollo 13. On TV, he appeared in "The Golden Girls," "21 Jump Street," "The Adventures of Brisco County Jr." "Mad About You" and as Abel Koontz on "Veronica Mars."
In 2006, he played real-life Sept. 11 victim Tom Burnett in United 93. The same year, he won a Best Guest Actor Emmy for his portrayal of Jerry "Hands" Espenson, an attorney with Asperger's syndrome on "Boston Legal."