Hordern was born in Berkhamsted, Hertfordshire, and educated at Brighton College, as was his brother Peter. He acted at school and then as an amateur with the St. Pancras People's Theatre before going into the profession. In 1937, he made his professional stage début, playing a minor role in Othello, and he appeared in many Shakespearean productions after World War II, taking roles such as Polonius in Hamlet and Malvolio in Twelfth Night.

For much of his career, Hordern was mainly known for comic roles, such as Mr Toad in Toad of Toad Hall on stage, but in his later years his King Lear was much acclaimed. He made more than a hundred film appearances, usually in character roles, including Passport to Pimlico (1949), Sink the Bismarck! (1960), El Cid (1961), The VIPs (1963), Khartoum (1966), Where Eagles Dare (1969), Anne of the Thousand Days (1969), England Made Me (1972), and Shogun (1980).

Hordern was also in demand for voice-over work, and as the narrator of FilmFair Production's Paddington Bear, his soothing voice will be familiar to TV audiences everywhere. He also provided the ironic voice-over narration in Stanley Kubrick's film Barry Lyndon, and can be heard in the adaptation of Richard Adams' Watership Down.

On radio he played Gandalf in the BBC's 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 famous indefatigable butler Jeeves in several series in the 1970s.

On television, he played Tartuffe for the BBC and appeared in several classic drama serials, one of his last performances being in Middlemarch (1994).

In later years, he had a house on Dartmoor, where he enjoyed fly fishing.

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.