In 1957, at the age of 27, Blakely made his stage debut as Dick McCardle in Master of the House. He also appeared in several Ulster Group Theatre productions, including Gerard McLarnon's Bonefire (1958) and Patricia O'Connor's A Sparrow Falls (1959). From 1957 to 1959 he was at the Royal Court Theatre, appearing in Cock-A-Doodle Dandy, Serjeant Musgrave's Dance and, to critical approval, The Naming of Murderers Rock. In 1961, he joined the Royal Shakespeare Company at Stratford-upon-Avon and from 1963 to 1968 was with the National Theatre at the Old Vic.
Film roles included Maurice Braithwaite in This Sporting Life (1963), Vahlin in The Long Ships, Dr. Watson to Robert Stephens's Holmes in The Private Life of Sherlock Holmes (1970), and Joseph Stalin in Jack Gold's Red Monarch (1983). In the 1975 British film, It Shouldn't Happen to a Vet, derived from the James Herriot books, Blakely played the eccentric Siegfried Farnon. He also appeared in A Man for All Seasons (1966), Young Winston (1972), The National Health (1973), Murder on the Orient Express (1974), The Pink Panther Strikes Again (1976), Equus (1977), The Dogs of War (1980), Nijinsky (1980) and Evil Under the Sun (1982).
In 1969, Blakely's controversial role as Jesus Christ in Dennis Potter's Son of Man gained him wide recognition. From that time onwards, he was a regular on British television, and in the same year played the leading role in a BBC adaptation of Trollope's The Way We Live Now.
Among the many stage plays in which he appeared were The Recruiting Officer, Saint Joan, The Royal Hunt of the Sun, Filumena Marturano, Volpone and Oedipus. He returned to the Royal Shakespeare in 1972 in Harold Pinter's Old Times and was subsequently in many West End plays.
A noted Shakespearean actor, Blakely appeared on television as Antony in Antony and Cleopatra (1981), directed by Jonathan Miller as part of the BBC Television Shakespeare series; and as Kent in the 1983 Granada Television version of King Lear which starred Laurence Olivier. Other television appearances included Loophole (1981), The Beiderbecke Affair (1985), Operation Julie (1985) and Paradise Postponed (1986).
