Williamson has worked as an actor on stage, television, and in film. He appeared in the Broadway premiere of Eric Bogosian's Talk Radio, starring Liev Schreiber, which ran from March 11 to June 24, 2007. Other stage performances include the Off-Broadway premiere of Przemyslaw Wojcieszek's Made in Poland. For this performance in Made in Poland, Williamson earned a positive review in Variety: "Williamson's hilariously serious perf as an angst-filled wannabe revolutionary strikes exactly the right note. As he wanders around his little town in post-Communist Poland, vandalizing cars and trashing phone booths, the unfairness of everything becomes so oppressive he delivers Bogus' every line like it's a prelude to a fistfight."