Since his movie debut in 1994's Dumb and Dumber, he has appeared in numerous movies and television shows including Enemy of the State, Half Baked, Freddy Got Fingered, Superstar, The Geena Davis Show, Down Periscope, Big Money Hustla$, Employee Of The Month, and Sorority Boys with cameos in many others, like My Name Is Earl and There's Something About Mary. He also starred in the Disney movie RocketMan. Among his other performances, in early 2001 he also provided the voice of the lustful, yet fun-loving Michael Timothy Bonner in the claymation UPN show "Gary & Mike" along with Christopher Moynihan providing the voice of the good-hearted nerd, Gary Newton. He later provided the voice of Lug in 2005's animated film Robots.
He has performed his stand up comedy numerous times at the Montreal comedy festival Just for Laughs, and other notable venues. His stand up routine is often odd, with obscure references to topics like koalas and KFC. He also appeared in Holidaze as Albert the Turkey.
He hosted the Just for Laughs television show for the 1995-96 season.
Currently, Williams voice-tracks (records) a radio show in Los Angeles that is played weekday afternoons in Denver, Colorado on 103.5 The Fox. He has also made semi-frequent appearances on The Adam Carolla Show, Loveline, and Late Night with Conan O'Brien. He is a favoured guest on the Conan O'Brien show for his quirky antics and apparent on-screen chemistry with Conan.
He was married to actress Wendi Kenya. Kevin Hearn of Barenaked Ladies is his cousin, and Williams appeared in the band's video for "Falling for the First Time".
He also recently appeared in the 2006 comedy film Employee of the Month.
In March 2007, he appeared as Sheriff Rosco P. Coltrane in The Dukes of Hazzard: The Beginning. In April 2007, he appeared on Thank God You're Here, where he won the Blue Door Award.