Clemenson grew up in Humboldt, Iowa where he delivered the local newspaper in his youth. After middle school, he received a scholarship to study at Phillips Academy in Andover, Massachusetts. While there his father passed away, at age 58.
After graduating from Phillips Academy, he entered Harvard College where he acted in many plays. He also attended the Yale School of Drama. After graduation he moved to Los Angeles.
Clemenson has appeared in many films and television shows, including United 93, Armageddon, The Big Lebowski, Boston Legal, Veronica Mars and The Adventures of Brisco County Jr.