John Gielgud was born in Kensington in London to Kate Terry and Frank Gielgud (who was of a Polish or Lithuanian Roman Catholic background), although he was not baptized or raised a Catholic. Gielgud had a head start in the theatrical profession, being a great nephew of Dame Ellen Terry.

After Westminster School, where he gained a King's Scholarship, he trained at RADA and had his initial success as a stage actor in classical roles. He starred and directed in many Royal Shakespeare Company productions at Stratford-upon-Avon. His Hamlet of 1936 was particularly admired and widely acclaimed.

Although he began to appear in British films as early as the 1930s, he would not make an impact in the medium until the last decades of his life. His film roles included: Benjamin Disraeli in The Prime Minister (1940), Cassius in Julius Caesar (1953) and George, Duke of Clarence to Laurence Olivier's Richard III (1955). Unlike Olivier, he remained primarily a stage actor, and so the rivalry between them was minimal.

As he aged, Gielgud began to adapt more to changing fashions in the theatre, appearing in plays by David Storey (Home), Charles Wood (Veterans), Edward Bond (Bingo) and Harold Pinter (No Man's Land). In the 1980s and 1990s, 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 his performance in Shine (1996) was critically acclaimed. Gielgud was also one of the few people who has won an Emmy, a Grammy, an Oscar and a Tony Award.

He was knighted in the 1953 coronation honours, became a Companion of Honour in 1977, and was admitted to the Order of Merit in 1996.

He was convicted of lewd behaviour (cottaging) in 1953. Instead of being rejected by the public, he got a standing ovation at his next stage appearance, and the roller-coaster to decriminalise homosexuality in England and Wales began. Longtime lover Martin Hensler, 30 years his junior, died just a few months before Sir John did in 2000.

His final acting performance was in a film adaptation of Samuel Beckett's short play Catastrophe, alongside longtime collaborator Harold Pinter; Gielgud passed away mere weeks after production was completed at the age of 96 of natural causes.

Sir John Gielgud believed that animals should not be exploited. He was particularly fond of birds and joined PETA's campaign against the foie gras industry in the early 1990s, narrating PETA's video exposé of the force-feeding of geese and ducks. Many chefs and restaurateurs who saw that video dropped foie gras from their menus. Sir John received PETA’s Humanitarian of the Year Award twice, in 1994 and 1999.