Writer, poet and playwright, now best known for preparing the libretto for Benjamin Britten's opera The Rape of Lucretia.

Duncan was born, with the surname Dunkelsbühler, in Salisbury, Southern Rhodesia (now Harare, Zimbabwe), in 1914. He became a pacifist during the 1930s, and his first publication, in January 1937, was The Complete Pacifist, a pamphlet appearing from the Peace Pledge Union (PPU) and carrying endorsements by Canon Dick Sheppard, Gerald Heard, and Sylvia Townsend Warner. Later that year he wrote the words for a Pacifist March composed by Benjamin Britten for the PPU, but the work was not a success and was soon withdrawn. In the same year also he visited Gandhi in India, and from 1938 was on friendly terms with the British Hispanist Gerald Brenan.

In 1937, again, Duncan met Ezra Pound, who encouraged him to found the "little magazine" Townsman, 1938-1945. Of the 24 issues, numbers 21-24 (1944–45) appeared as The Scythe, a title that signaled Duncan's increasing interest in agriculture and husbandry. His pacifism had led him to set up a co-operative farming enterprise at Mead Farm, near Welcombe, Devon, during the Second World War. This failed by 1943, and in 1944 Duncan successfully faced a conscientious objection tribunal. In 1942-43 he helped Britten with the last scene of the opera Peter Grimes, and wrote the whole of the libretto for The Rape of Lucretia in 1945-46.

Duncan's play This Way to the Tomb was performed at the Mercury Theatre, Notting Hill Gate in 1945, and was followed by his adaptation of Cocteau's L’aigle à deux têtes as The Eagle has Two Heads (1946). Tallulah Bankhead and Marlon Brando appeared in the U.S. production. Stratton was published in 1950. Our Lady's Tumbler was performed in Salisbury Cathedral for the Festival of Britain in on 5 June 1951, in front of the 7th Earl and Countess of Harewood. Don Juan was first performed in 1953, and The Death of Satan: a comedy in 1954. A joint production of the two latter plays was presented by the English Stage Company at the Royal Court Theatre in 1956, directed by George Devine. In 1962 there was controversy over the refusal of the Lord Chamberlain to permit public performance of The Catalyst, a play about a ménage à trois. These verse plays in the manner of T. S. Eliot became less popular from the mid-1960s.

Ronald Duncan was instrumental in setting up and naming the English Stage Company at London's Royal Court Theatre, which opened in 1956. 

Duncan was also a writer of short stories and a journalist. He wrote the film script for Girl on a Motorcycle (dir. Jack Cardiff, 1968), which starred Marianne Faithfull. His poetry was published at Faber and Faber by T. S. Eliot, who became a friend.

In 1964 Duncan published All Men are Islands, the first of a series of lively and sometimes contentious and contradictory autobiographies. How to Make Enemies followed in 1968, and Obsessed in 1977. A final controversial autobiography, Working with Britten: A Personal Memoir appeared from Duncan's own Rebel Press in 1981 after being rejected by a mainstream publisher.

In the late 1960s and early 1970s he worked on a long poem about science, Man, in five parts (1970–74), and in 1978 he co-edited The Encyclopedia of Ignorance with Miranda Weston-Smith.

Duncan died in hospital at Barnstaple, Devon, England, in 1982.