Films:
* The Andromeda Strain (1971) - Dr. Charles Dutton
* The Towering Inferno (1974) - Jim Harris
* The Shootist (1976) - Mayor
* The Swarm (1978) - Dr. Robert Hoag
* The Black Hole (1979) - Captain Alex Durant
* The Fog (1980) - Father Malone
* The Last Starfighter (1984) - Centauri
* The Jewel of the Nile (1985) - Dr. Omar
* The Dead Pool (1988) - Mr. Big
* The Wizard (1989) - The Wizard
* The Rocketeer (1991) - Packard
* The Santa Clause (1994) - Santa Claus
* Batman & Robin (1997) - Mr. Freeze (voice)
* The Faculty (1998) - Principal
* The Patriot (2000) - Reverend
* The Scorpion King (2002) - Memnon
* The Chronicles of Riddick (2004) - Lord Marshal
* The Day After Tomorrow (2004) - President Blake
* The Last Mimzy (2007) - Mr. Abernathy
* Transformers (2007) - General Morshower
* Iron Man 2 (2010) - General (uncredited)
* The Green Hornet (2011) - Franklin D. Roosevelt (voice)
Television:
* The Wild Wild West (1966-1969) - Recurring role as Dr. Miguelito Loveless
* Mission: Impossible (1967-1973) - Recurring role as "The Chief"
* Mannix (1967-1975) - Recurring role as Mr. Big
* The Six Million Dollar Man (1974-1978) - Recurring role as Dr. Rudy Wells
* The Incredible Hulk (1978-1982) - Recurring role as General Ross
* Knight Rider (1982-1986) - Recurring role as Devon Miles
* Murder, She Wrote (1984-1996) - Recurring role as Sheriff Amos Tupper
* The A-Team (1983-1987) - Recurring role as Colonel Decker
* Star Trek: The Next Generation (1987-1994) - Recurring role as Admiral Eric Pressman
* Babylon 5 (1994-1998) - Recurring role as President Clark
* The X-Files (1993-2002) - Recurring role as The Smoking Man
* The Pretender (1996-2000) - Recurring role as Mr. Jones
* Walker, Texas Ranger (1993-2001) - Recurring role as Sheriff Bob McGruder
* JAG (1995-2005) - Recurring role as Admiral Chegwidden
* CSI: Miami (2002-2012) - Recurring role as Dr. Raymond Caine
This is not an exhaustive list, but it highlights some of Dan Seymour's most memorable roles. He was a highly respected actor who brought a great deal of gravitas to every role he played.