From 1935 to 1953 he attended St. Joseph's School and Campion College in Regina, Saskatchewan, where his acting career began under the direction of Rev. Arthur Nelson, S.J. and Mary Ellen Burgess at Regina Little Theatre. Vernon was educated at the Banff School of Fine Arts and the Royal Academy of Dramatic Art in London before becoming a live stage actor for CBC Television's dramatic programs. In 1974, he completed a season at The Royal Shakespeare Theatre in Stratford-Upon-Avon, England, playing Malvolio.

Vernon made his screen debut in 1956 as the voice of Big Brother in Michael Anderson's film version of George Orwell's 1984 starring Edmond O'Brien. He returned to Canada afterwards and gained film experience appearing on the TV series Tugboat Annie and The Last of the Mohicans.

He made his Broadway debut in 1964 as DeSoto opposite Christopher Plummer and David Carradine in The Royal Hunt of the Sun. During the Golden Age of CBC Drama in the 1960s he co-starred in Edna O'Brien's A Cheap Bunch of Nice Flowers, opposite Colleen Dewhurst, and in Uncle Vanya, opposite william Hutt and Rita Gam. He appeared in the CBC series Wojeck in the late 1960s, playing a crime-fighting medical examiner, but left to further his acting career in the United States.

In 1967, he appeared opposite Lee Marvin in Point Blank. In 1969, he played Cuban revolutionary Rico Parra in Alfred Hitchcock's Cold War-era spy movie Topaz. He appeared on The High Chaparral as the leader of a group of striking Irish Miners (1969) in "No Irish Need Apply". In 1970, he guest-starred in the Hawaii Five-O episode "Force Of Waves" as Cal Anderson, and he appeared in the two-part episode "The Banker" of The Silent Force in 1971. In the late 1960s and early 1970s, he made four appearances over five years on the TV series Mission: Impossible as four different lead villains. In 1974, Vernon turned in a supporting performance in Mary Jane Harper Cried Last Night.

In 1971, he played the by-the-book mayor of San Francisco, perpetually frustrated by Clint Eastwood, in the first Dirty Harry movie. He later parodied this role in the premiere episode of Sledge Hammer! and One More Train to Rob. In 1972 he appeared as a villain in the Fear Is the Key. In 1973 he appeared in Charley Varrick.. In 1974 he co starred in the film The Black Windmill with Michael Caine and Donald Pleasence. Also in 1974, he appeared in The Questor Tapes. In 1975, he starred in Brannigan, alongside John Wayne and Richard Attenborough. In 1976, he played Fletcher in Eastwood's The Outlaw Josey Wales.

In 1979, Vernon played villainous American bigot Jonathon Pritts in New Mexico who was trying to steal the land from Mexican landowners in the Louis L'Amour story of The Sacketts.

He also appeared in several cult exploitation and action films in the 1980s, most notably Chained Heat and Savage Streets, both starring Linda Blair, and Jungle Warriors, opposite Sybil Danning. He made light of his villain image in the 1988 Blaxploitation spoof I'm Gonna Git You Sucka: a character thinks Vernon should be "above exploitation films" and Vernon replies that lots of famous actors have done exploitation films, listing Jamie Lee Curtis, Angie Dickinson, and Shelley Winters as examples.

Vernon played "Ted Jarrett" in the season two The A-Team episode "Labor Pains" (1983). Vernon also played "Cameron Zachary" in the season two Knight Rider episode "A Good Knight's Work" (1984). Vernon later played "John Bradford Horn" in the season three Airwolf episode "Discovery" (1986). In 1986, he played the Principal in Fuzz Bucket. He played Sergeant Curt Mooney in Killer Klowns from Outer Space and was a lead in the short-lived 1990s series Acapulco H.E.A.T. In Charley Varrick (1973) he played a mafia boss.

Vernon died of complications following heart surgery on February 1, 2005, in Westwood, California. He was cremated after a private funeral Service.

Vernon was born Adolphus Raymondus Vernon Agopsowicz in Zehner, Saskatchewan, and was baptised at Sacred Heart Catholic parish in the nearby town of Arat. He was one of two sons of Adolf Agopsowicz, a grocer, and his wife Eleonore Krückel (also spelled as Kriekle or Kriekel). Both parents' families immigrated to the Edenwold district in the late 19th century from the Austrian crownland and duchy of Bukovina. The Agopsowicz family were part of the community of Armenians in Poland. Vernon was of Armenian, German, and Polish descent.