He is from a distinguished acting family on his mother's side, including his great-grandfather Herbert Standing (1846–1923) and his grandfather, Sir Guy Standing (1873–1937). His stepfather was actor Sir John Clements. He was educated at Eton College and Millfield School, Somerset, later serving in the King's Royal Rifle Corps as a second lieutenant, before going to study at the Byam Shaw School of Art in London.
He began his career in Peter Brook's 1955 production of Titus Andronicus starring Laurence Olivier and wife Vivien Leigh and later played leading parts in Oscar Wilde's The Importance of Being Earnest, Ring Around the Moon, A Sense of Detachment by John Osborne, and Noël Coward's Private Lives, with Maggie Smith. He was nominated for an Olivier award (1979) for Close Of Play at the National Theatre. He made his film debut in The Wild and the Willing (1962), going on to appear in King Rat (1965), Walk, Don't Run (1966), The Psychopath (1966), The Eagle Has Landed (1976), The Elephant Man (1980), Nightflyers (1987), Mrs. Dalloway (1997), and A Good Woman (2004).
He succeeded his father as the 4th baronet in 1964, but does not use the title. The Leon family were, until 1937, owners of Bletchley Park, the country house in Buckinghamshire used in the Second World War as a code-breaking centre. His half-sister was the Actress Anne Leon, who was married to Michael Gough. He married Jill Melford in 1961; the couple had a son, but divorced in 1972.
One of his first major television roles was as Sidney Godolphin in the BBC twelve part serial, The First Churchills (1969). Other television appearances include Tinker, Tailor, Soldier, Spy (1979); the ITV sitcom The Other 'Arf (1980–84), with Lorraine Chase; The Choir (1995) and King Solomon's Mines (2004). In the USA, he made guest appearances in numerous weekly programmes including L.A. Law, Civil Wars and Murder, She Wrote, and co-starred briefly with Robert Wagner and Samantha Smith in the action series Lime Street (1985).
He appeared in the 1987 film Nightflyers adapted from a short story by George R. R. Martin. In July 2010 it was confirmed that he would be appearing as Jon Arryn in the HBO series Game of Thrones, based on Martin's A Song of Ice and Fire novels.
