He then moved to the U.S. after the war and made a career teaching.
After starring in Frasier, Mahoney moved back to Chicago and acted with the Steppenwolf Theatre once again.
He also famously appeared on The Simpsons alongside his Frasier co-stars, Kelsey Grammer and Hyde Pierce, playing Robert Terwilliger, Sr. - the father of Sideshow Bob (played by Grammer) and Cecil Terwilliger (Hyde Pierce).
Born in Blackpool, England, Mahoney came to the US when he was 19.
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After a brief stint in the USA army, Mahoney became an English teacher, and later the editor of a medical journal.
Mahoney's portrayal of Martin earned him two Emmy nominations, two Golden Globe nominations and a Screen Actors Guild award.
Prior to becoming a theater actor in the late 1970s, Mahoney worked as an editor and, dissatisfied with his career, began taking acting classes at age 37.
In 1986, he won a Tony Award for his role in John Guare's The House of Blue Leaves. In 2007, he returned to Broadway for a revival production of Craig Lucas' Prelude to A Kiss.
With the 1987 release Tin Men, Mahoney made his feature film debut. His subsequent roles included the manager of the corrupt White Sox baseball team, Kid Gleason, in Eight Men Out (1988), and Ione Sky's embezzling father in Say Anything (1989), directed by Cameron Crowe. He also played in Moonstruck, Say Anything and Barton Fink. Remember him well." Wendie Malick, whose character Ronee married Martin in the sitcom's final episode, posted a picture of herself and John on set, and tweeted: "John Mahoney was not only a brilliant actor on stage & screen, but a ideal gentleman.
He most recently appeared on TV screens in 2015, when he portrayed businessman Andrew Del Mar in an episode of ITV drama Foyle's War.