He was a British writer and publisher. As a child, he had a cloak made of rabbit skin and thus received the nickname "Bunny", by which he was known to friends and intimates all his life. Garnett was born in Brighton, the only child of the writer, critic and publisher Edward Garnett and his wife Constance, a translator of Russian. As a conscientious objector in the First World War, David worked on fruit farms in Suffolk and Sussex with his lover, Duncan Grant. A prominent member of the Bloomsbury Group, Garnett received literary recognition when his novel Lady into Fox, an allegorical fantasy, was awarded the 1922 James Tait Black Memorial Prize for fiction. He ran a bookshop near the British Museum with Francis Birrell during the 1920s. He also founded (with Francis Meynell) the Nonesuch Press. He wrote the novel Aspects of Love (1955), on which the later Andrew Lloyd Webber musical was based. His first wife was illustrator Rachel "Ray" Marshall (1891–1940), sister of translator and diarist Frances Partridge. He and Ray, whose woodcuts appear in some of his books, had two sons, one of whom (Richard) went to Beacon Hill School. Ray died relatively young of breast cancer. Garnett was bisexual, as were several members of the artistic and literary Bloomsbury Group, and he had affairs with Francis Birrell and Duncan Grant. He was present at the birth of Grant's daughter, Angelica (by Vanessa Bell, and accepted by her husband Clive Bell), on 25 December 1918, and wrote to a friend shortly afterwards, "I think of marrying it. When she is 20, I shall be 46 – will it be scandalous?". When Angelica was in her early twenties, they did marry (on 8 May 1942), to the horror of her parents. The Garnetts lived at Hilton Hall, Hilton near St Ives in Cambridgeshire, where David Grant kept a herd of Jersey cows. They had four daughters: in order, Amaryllis, Henrietta, and twins Nerissa and Frances; eventually the couple separated. Amaryllis Garnett (1943–1973) was an actress who had a small part in Harold Pinter's film adaptation of The Go-Between (1970). She drowned in the Thames, aged 29. Henrietta Garnett married Lytton Burgo Partridge, her father's nephew by his first wife Ray, but was left a widow with a newborn infant when she was 18; she oversaw the legacies of both David Garnett and Duncan Grant. Nerissa Garnett (1946-2004) was an artist, ceramicist, and photographer. Fanny Garnett moved to France where she became a farmer.

After his separation from Angelica, Garnett moved to France and lived in a pleasant house in the grounds at the Château de Charry, Montcuq (near Cahors) leased to him by the owners, Jo and Angela D'Urville. He continued to write, made friends among the local English community of the locality, and lived there until his death in 1981. According to a recent biography, "Bloomsbury's Outsider: A Life of David Garnett" by Sarah Knights, David Garnett's body was donated to the School of Anatomy in Bordeaux by his request.