[5] He has appeared in a number of highly acclaimed films, including Hannah and Her Sisters, Broadcast News, Apollo 13 and The Big Lebowski, and portrayed Tom Burnett in Paul Greengrass' United 93.
[1] In his early teens, he delivered the Des Moines Register, which had a long tradition of awarding scholarships to top East Coast preparatory schools to a select few "paper boys.
"[6] Clemenson, a straight-A junior high school student, won a Register scholarship to Phillips Academy in Andover, Massachusetts.
[11] He displayed early on his remarkable range when he undertook with inimitable flair and hilarious effect the hugely solemn role of Erde "the green-faced torso", goddess of the Earth, in Peter Sellars's Loeb Drama Center puppet production of Wagner's Ring cycle (1979).
He had a regular role as Socrates Poole, a lawyer in the Old West, in the Fox series The Adventures of Brisco County, Jr.
In 1999, Clemenson appeared in the Buffy the Vampire Slayer episode "Bad Girls" as a grotesquely obese demon named Balthazar.
Clemenson starred in the film United 93 as Tom Burnett, one of the victims of the 9/11 hijacking of the flight that crashed in rural Pennsylvania after a passenger revolt.