Films
* Die Hard (1988) - as Hans Gruber's right-hand man, "Karl" - Arguably his most famous role, this film launched him into mainstream recognition.
* The Jewel of the Nile (1985) - as "Ralph" - Another action-comedy alongside Michael Douglas and Kathleen Turner
* Ghostbusters (1984) - as "Walter Peck" - The environmental inspector who gets into a heated debate with Peter Venkman.
* The Godfather Part III (1990) - as "Willis" - A corrupt lawyer.
* The Bedroom Window (1987) - as "Terry Lambert" - A film noir thriller.
* The Seduction of Joe Tynan (1979) - as "Richard 'Dick' Stockton" - A political drama.
* The Day of the Locust (1975) - as "Tod Hackett" - His breakout role in this film adaptation of the Nathanael West novel.
Television
* The West Wing (2000-2006) - as "Leo McGarry" - A recurring role as the White House Chief of Staff
* The Twilight Zone (1985-1989) - as "Charles" in the episode "The Shadow Man"
* The Equalizer (1985-1989) - as "Dr. Evan Thorn"
* Superman (1978-1987) - as "Henry Morgan"
* The Rockford Files (1974-1980) - as "Jack" in the episode "The Last Day"
* Murder, She Wrote (1984-1996) - as "Dr. Mark Peters"
This is just a sampling of his many projects. William Atherton has been a consistently working actor, and his performances have left a mark on cinema and television.
To get a more complete list of his filmography, you can consult resources like IMDb or Rotten Tomatoes.