Arthur John Gielgud was born in South Kensington in London to a Protestant mother, Kate Terry, and a Catholic father, Frank Gielgud, and was raised a Protestant. Gielgud had a head start in the theatrical profession, being a great nephew of Dame Ellen Terry. His elder brother was Val Gielgud who was a pioneering influence in BBC Radio. His niece is Maina Gielgud, dancer and one time artistic director of The Australian Ballet and the Royal Danish Ballet.

After Westminster School, where he gained a King's Scholarship, Gielgud trained at RADA and had his initial success as a stage actor in classical roles, first winning stardom during a successful two seasons at the Old Vic Theatre from 1929 to 1931 where his performances as Richard II and Hamlet were particularly acclaimed, the latter being the first Old Vic production to be transferred to the West End for a run.

Gielgud returned to the role of Hamlet in a famous production under his own direction in 1934 at the New Theatre in the West End. He was hailed as a Broadway star in Guthrie McClintic's production in which Lillian Gish played Ophelia in 1936 (and which was assisted by a rival staging starring Leslie Howard that opened shortly afterwards and failed badly by comparison), a 1939 production that Gielgud again directed that was the last play performed at Henry Irving's Orpheum Theatre and was later taken to Elsinore Castle in Denmark (the actual setting of the play), a 1944 production directed by George Rylands, and finally a 1945 production that toured the Far East under Gielgud's own direction. In his later years, Gielgud would play the Ghost of Hamlet's Father in productions of the play, first to Richard Burton's Melancholy Dane on the Broadway stage which Gielgud directed in 1964, then on television with Richard Chamberlain, and finally in a radio production starring Gielgud's protégé Kenneth Branagh.

Gielgud had triumphs in many other plays, notably his greatest popular success Richard of Bordeaux (1933) (a romantic version of the story of Richard II), The Importance of Being Earnest which he first performed at the Lyric Hammersmith in 1930 and which would remain in his repertory until 1947, and a legendary production of Romeo and Juliet (1935) which Gielgud directed and alternated the roles of Romeo and Mercutio with a young Laurence Olivier in his first professional Shakespearean leading role.

Olivier's performance won him an engagement as the leading man of the Old Vic Theatre the following season, starting his career as a classical actor, but he was said to have resented Gielgud's direction and developed a wary relationship with Gielgud which resulted in Olivier turning down Gielgud's request to play the Chorus in Olivier's film of Henry V and later doing his best to block Gielgud from appearing at the Royal National Theatre when Olivier was its director.

Although he began to appear in British films as early as 1924, making his debut in the silent movie Who Is the Man?, he would not make an impact in the medium until the last decades of his life. His early film roles were sporadic and included the lead in Alfred Hitchcock's Secret Agent (1936), Benjamin Disraeli in The Prime Minister (1940), Cassius in Julius Caesar (1953) (BAFTA Award for Best British Actor), George, Duke of Clarence to Olivier's Richard III (1955), and Henry IV to Orson Welles' Falstaff in Chimes at Midnight (1966).

But he lost his aversion to filming in the late 1960s, and by the 1980s and 1990s he had thrown himself into the medium with a vengeance, so much so that it was jokingly said that he was prepared to do almost anything for his art. He won an Academy Award for his supporting role as a sardonic butler in the 1981 comedy Arthur, starring Dudley Moore and Liza Minnelli, a New York Film Critics Circle Award for Providence (1977), and a BAFTA Award for Murder on the Orient Express (1974), and his performances in The Charge of the Light Brigade (1968), The Elephant Man (1981), and Shine (1996) were critically acclaimed. In 1991, Gielgud was able to satisfy his life's ambition by immortalizing his Prospero on screen in the film Prospero's Books.

Television also developed as one of the focal points of his career, with Gielgud giving a particularly notable performance in Brideshead Revisited (1981). He won an Emmy Award for Summer's Lease (1989) and televised his stage performances of A Day by the Sea (1957), Home (1970), No Man's Land (1976) and his final theatre role in The Best of Friends as Sydney Cockerell in the 1991 Masterpiece Theatre Production, along with Patrick McGoohan and Dame Wendy Hiller.

In 1983, he made his second onscreen appearance with fellow theatrical knights Laurence Olivier and Ralph Richardson (following Olivier's own Richard III) in a television miniseries about composer Richard Wagner. In 1996 he played a wizard in the TV adaptation of Gulliver's Travels.

Gielgud and Ralph Richardson were the first guest stars on Second City Television. Playing themselves, they were in Toronto during their tour of Harold Pinter's No Man's Land. According to Dave Thomas, in his book, SCTV: Behind the Scenes, their sketch stank and the actors gave a bad performance. Gielgud's final television performance was on film in Merlin in 1998, his final television studio appearance having been in A Summer Day's Dream recorded in 1994 for the BBC 2 Performance series.

Gielgud was one of the few people who has won an Emmy, a Grammy, an Oscar and a Tony Award.

Gielgud's final onscreen appearance in a major release motion picture was as Pope Paul IV in Elizabeth which was released in 1998. His final acting performance was in a film adaptation of Samuel Beckett's short play Catastrophe, opposite longtime collaborator Harold Pinter and directed by American playwright David Mamet; Gielgud died mere weeks after production was completed at the age of 96 of natural causes.