Hordern was born in the Poplars, an 18th-century townhouse in Berkhamsted, Hertfordshire, the son of Margaret Emily (Murray) and Capt. Edward Joseph Calverly Hordern. He was educated at Windlesham House School and Brighton College, as was his elder brother Peter, who played rugby for Gloucester in the 1930s and was capped four times by England. He acted at school and then as an amateur with the St Pancras People’s Theatre. He worked as a schoolteacher and travelling salesman before becoming a professional actor. In 1937 he made his professional stage debut at the People’s Palace, East London, playing a minor role in Othello and, later in the year, joined the repertory company of the Little Theatre in Bristol. There he met the actress Grace Eveline Mortimer; they married in 1943 and remained together until her death in 1986. They had one daughter, Joanna.  Hordern received a commission into the Royal Navy and went on to serve on HMS Illustrious, taking part in Operation Ironclad in 1942. He also held a post with the rank of Lieutenant-Commander in the office of the First Sea Lord. His stage work, in Stratford for the Royal Shakespeare Company and in London at the Old Vic and in the West End, demonstrated his wide range and distinctive, rich voice. In addition to his many Shakespearean roles (Jaques in As You Like It, Cassius in Julius Caesar, Polonius in Hamlet, Malvolio in Twelfth Night), Hordern performed in plays by Strindberg, Chekhov, Ibsen, Pinero, Pinter, Dürrenmatt, Albee, Alan Ayckbourn, David Mercer and Tom Stoppard.  He played the title role in King Lear, directed by Jonathan Miller, at the Nottingham Playhouse in 1970. He reprised the role for Miller on two further occasions, in 1975 and in the BBC Television Shakespeare series in 1982. In 1978 he returned to Stratford to play a wise Prospero in The Tempest. This was also replicated for the BBC Shakespeare series in 1980.

He made more than 160 film appearances, usually in character roles, including Passport to Pimlico (1949), Scrooge (1951, as Jacob Marley; he was to play Ebenezer Scrooge himself in a 1977 TV adaptation), The Heart of the Matter (1953), Grand National Night (1953),The Spanish Gardener (1956), Sink the Bismarck! (1960), El Cid (1961), Cleopatra (1963), The V.I.P.s (1963), The Spy Who Came in from the Cold (1965), Khartoum (1966), A Funny Thing Happened on the Way to the Forum (1966), The Taming of the Shrew (1967), Where Eagles Dare (1969), Anne of the Thousand Days (1969), England Made Me (1972), Alice’s Adventures in Wonderland (1972), Juggernaut (1974), The Slipper and the Rose (1976), Shogun (1980) and Gandhi (1982). In 1968 he appeared as the central character in Jonathan Miller’s television adaptation of M.R. James’s ghost story Whistle and I’ll Come to You. Some years later Hordern narrated nineteen unabridged supernatural stories by M.R. James, released across four audio cassette collections by Argo Records in the 1980s. In 1986 he appeared in the TV series Paradise Postponed. In 1992 he narrated the two-cassette recording of the John Mortimer story Rumpole on Trial. Mortimer had originally wanted Horden to play the role of Rumpole in the character’s original appearance in a 1974 television play, but Horden was unavailable at the time, so the role went to Leo McKern.

Hordern was also in demand for other voice-over work. He was the narrator of FilmFair Productions’ Paddington, and was the voice of Badger in the 1980s TV series The Wind in the Willows. He also provided the ironic voice-over narration in Stanley Kubrick’s film Barry Lyndon, and can be heard playing the part of the rabbits’ god Frith in Martin Rosen’s animated adaptation of Richard Adams’ Watership Down (1978).  On radio he played Gandalf in the BBC radio adaptation of Tolkien’s Lord of the Rings (1981); another great wizard, Merlin, in an adaptation of T.H. White’s The Sword in the Stone (1982); and P.G. Wodehouse’s valet Jeeves in several series in the 1970s and early 1980s. Hordern was the reader for an abridged 1981 recording of C.S. Lewis’ The Chronicles of Narnia, and was the narrator of several radio adaptations of Ellis Peters’ Brother Cadfael novels.

On television Hordern played Tartuffe for the BBC’s Play of the Month series in 1971 and Professor Marvin in The History Man in 1980. He also appeared in several classic drama serials, his last performance being in the BBC adaptation of Middlemarch (1994).  He had bought a house in Bagnor, near the town of Newbury, Berkshire in 1956, and spent his final years there. He enjoyed fishing on the River Lambourn which was close to the house, and where dramatist Tom Stoppard “shared a rod” with him (as Stoppard once put it).  Hordern was appointed CBE in 1972 and knighted in 1983. He died as a result of kidney disease in May 1995, at the age of 83. Shortly before his death, Brighton College named a room in his honour where a bronze portrait bust stands; the National Portrait Gallery in London has another copy.