Gielgud was the grandnephew of the celebrated English actress Ellen Terry (Terry, Ellen). He was educated at Westminster School and at the Royal Academy of Dramatic Art, London, and made his acting debut in 1921 at the Old Vic Theatre, London, later playing Romeo at the Regent Theatre, London, in 1924. He made his first American appearance in New York City in 1928. After affiliations with the Oxford Playhouse, he joined the Old Vic company, for which his performance in 1929 as Hamlet established his reputation as one of England's most promising actors. A series of impressive Shakespearean performances followed. His greatest early success was probably as Richard II in the play by that name, which he also directed.

An actor of considerable versatility with a superbly controlled speaking voice, Gielgud performed in such diverse plays as Richard Brinsley Sheridan's School for Scandal, Oscar Wilde's The Importance of Being Earnest, Anton Chekhov's The Seagull, Graham Greene's The Potting Shed, and Edward Albee's Tiny Alice. He directed the repertory seasons of 1937–38 at the Queen's Theatre, London, and of 1944–45 at the Haymarket Theatre, London.

Gielgud's writings include his autobiography Early Stages (1938; rev. ed., 1976); Stage Directions (1963), a collection of speeches and essays; Distinguished Company (1972), detailing some of his “youthful enthusiasms” for stars of stage and screen; an amply illustrated memoir, Gielgud: An Actor and His Time, with John Mills and John Powell (1980); and Shakespeare: Hit or Miss? (1991; also published as Acting Shakespeare, 1992), with John Miller, reminiscences and observations on his Shakespeare acting and directing.