His six decades of television credits, he is best known for playing Burke Devlin in the 1960s gothic soap opera Dark Shadows, and later for his co-starring role as Thomas Gibson's father Edward Montgomery on Dharma & Greg.

[2] A life member of the Actors Studio,[4] Ryan's Broadway theatre credits include Wait Until Dark, Medea, and The Price.

[6] Ryan was an original cast member of the cult TV soap opera Dark Shadows, playing Burke Devlin until he was dismissed from the show in June 1967 due to his alcoholism,[7][8][9] and replaced by Anthony George.

[10]: 549 Ryan portrayed Cooper Hawkins on the Western series The Chisholms (1980),[10]: 185–186  Sam Garrett on King's Crossing (1982)[10]: 567  Brennan Flannery on High Performance (1983),[10]: 459  Edward Wyler on Hot Pursuit (1984),[10]: 478  and Porter Tremont on 2000 Malibu Road (1992).

[10]: 1122–1123 His other acting credits include the films Liar Liar; Magnum Force playing as Dirty Harry's ill-fated despondent best friend and fellow police officer, a motorcycle patrolman named Charlie McCoy; Lethal Weapon playing the key villain General Peter McAllister; Grosse Pointe Blank; Electra Glide in Blue; and Hot Shots!

Ryan appeared in NBC's The A-Team; he played Ike Hagan, as Grant Everett in a two-part Silk Stalkings episode; and as Kyle Riker, the father of Commander William Riker, in the Star Trek: The Next Generation episode "The Icarus Factor"; Ryan had been considered for the part of series lead Captain Jean-Luc Picard.