Ryan was born in Bayswater, London and trained at East 15 Acting School from 1968 to 1971. He began his professional acting career with Glasgow Citizens' Theatre in 1971. He is of Greek descent.

Although his film appearances were few, he played small roles in Santa Claus: The Movie (1985) and the 1993 film Dirty Weekend, directed by Michael Winner.

In 1986, he appeared in the science fiction series Doctor Who as the alien creature Lord Kiv in the serial The Trial of a Time Lord: Mindwarp, and as one of the rose-painting playing cards in the 1999 TV version of Alice in Wonderland. In 2008, he again guest-starred in Doctor Who in the episodes "The Sontaran Stratagem" and "The Poison Sky", as the Sontaran leader "General Staal", and in the 2010 episode "The Pandorica Opens", playing another Sontaran character, "Commander Stark". In 2012 he appeared in the role of Marshall in Episode 3 "Olympics" of the Absolutely Fabulous 20th-anniversary specials.

In 1987, he starred in the short-lived BBC sitcom A Small Problem. He played Lucky in Samuel Beckett's Waiting for Godot at the Queen's Theatre in the West End in 1991, alongside Mayall and Edmondson. He later played Dave Hedgehog in Bottom, and Marshall Turtle, one of Edina's unloved ex-husbands, on Absolutely Fabulous. He has also appeared in two major sitcoms: he played one of the Driscoll brothers in an episode of Only Fools And Horses and spin-off The Green Green Grass, a pair of identical twin builders in the One Foot in the Grave episode "Hole In The Sky", and a plumber in the episode "The Valley of Fear". He appeared in Back to School Mr. Bean, My Family and Saxondale. He joined up again with Jennifer Saunders for two episodes of The Life and Times of Vivienne Vyle.

Ryan starred as Baron Hardup in the pantomime of Cinderella at the Theatre Royal, Norwich from 17 December 2013 to 19 January 2014.