Following education at Oxford University, Gielgud began his career as a secretary to a Member of Parliament, before moving into writing when he took a job as the sub-editor of a comic book / magazine. It was this job that led him to work for the BBC's own listings the magazine, the Radio Times, as the assistant to the editor Eric Maschwitz. This was Gielgud's first connection to the Corporation, and although he was not involved in any radio production, he often used his position at the magazine to make his thoughts on radio dramas felt: in his autobiography, he later confessed to having written several of the letters appearing on the magazine's correspondence page, supposedly from listeners, criticising various aspects of the Corporation's programming.

Maschwitz and Gielgud were close friends, and even wrote detective fiction together — Gielgud would later on go on to be responsible in whole or part for twenty-six detective / mystery novels, one short story collection, two historical novels, nineteen stage plays, four film screenplays, forty radio plays, seven non-fiction books and be the editor of a further two books.

In January 1929, Gielgud was appointed Head of Productions at the BBC, responsible for all radio drama, when he had never previously directed a single radio play. He succeeded R E Jeffrey, whose output he had been so regularly criticising in his abuse of the Radio Times letters page. He proved to be highly successful in this role, remaining in it for the next twenty years and overseeing all of the radio drama produced during the period, writing many plays himself and sometimes appearing as an actor in small parts.

Gielgud is often praised with inventing many of the techniques of radio drama still common in the form today. He constantly reminded those working with and under him that radio drama could employ vastly larger casts and place itself in more exotic settings than was possible with regular stage plays, and held a theory that while stage plays could show the actions of characters, in radio it was possible to get inside of their minds.

He was not a fan of the soap opera genre which was rising to prominence on radio in the United States at the time — instead he preferred to concentrate on producing a variety of one-off dramas, rather than continuing series. He was not averse to producing the popular as well as the cultural, however, with various mysteries and thrillers being broadcast as well as more 'refined' productions such as Shakespeare plays.

In July 1930, Gielgud was invited in his position as the senior drama producer at BBC radio to oversee the experimental transmission of a short play on the new medium of television. The play, The Man With the Flower in His Mouth by Italian playwright Luigi Pirandello, was chosen because of its confined setting, small cast of characters and short length of around half an hour. Transmitted live on the evening of 14 July, the primitive state of television technology at the time allowed for only tiny 30-line pictures with one actor visible at a time, but the experiment was nonetheless judged to have been a success, and was even watched by the then Prime Minister, Ramsay MacDonald.

Gielgud remained in radio for the rest of the decade, also working occasionally in film, adapting his thriller Death at Broadcasting House, in which he also appeared in a small acting role. In 1939 he returned to television for a time on a secondment to the BBC Television Service at Alexandra Palace, which was now a fully-fledged, high-definition television network broadcasting to the London area. On secondment from his radio job, he produced one short play called Ending It, an adaptation of one of his own short stories starring John Robinson and Joan Marion, transmitted on 25 August 1939. However, a full-length play he was due to direct, and which had even been rehearsed, was cancelled from its planned slot on the evening of 1 September due to the television service having been suspended earlier that day in anticipation of the declaration of war.

Gielgud returned to radio for the duration of the Second World War, but shortly after the return of the television service in 1946, he moved across on a more permanent basis to become the Head of BBC television drama. Although the BBC hoped he would have the same impact on shaping the genre in the new medium as he had done in radio, his time in charge was not regarded as a success, as many of the producers working under him felt he had no great liking for television or appreciation for what it could achieve that radio could not. In 1952, he left the television service, being replaced by the experienced producer Michael Barry.

In the 1950s, Gielgud was involved in directing a run of Sherlock Holmes radio plays starring his brother John as the lead character, with Ralph Richardson as Dr Watson and Gielgud himself once appearing as Mycroft Holmes. These were broadcast on the BBC's Light Programme. By this time Gielgud was in conflict with junior colleagues in the drama department; unlike them he was unable to appreciate the work of playwrights such as Harold Pinter.

Gielgud was married five times, the first in 1921 while he was still an undergraduate at Oxford, where he married an eighteen year-old schoolgirl, whose name is recorded only as 'Tata', who was a student at Cheltenham Ladies College. In fact, Tata was Nathalie Mamontov (1903—1969), daughter of musician Sergei Mamontov (1877...1938) and Nathalie Sheremetievskya; her mother's third husband was Grand Duke Michael, brother of Tsar Nicholas II. This lasted for only two years, however, and they divorced in 1926.

His following four marriages produced two sons. He was also the grandfather of choreographer Piers Gielgud.

He published his autobiography in 1957, and died in London in 1981 at the age of eighty-one.