Galsworthy's sister Lilian (1864–1924) was married to the German Painter and lithographer Georg Sauter since 1894. With the beginning of World War I Sauter was interned as an enemy alien at Alexandra Palace and later expelled. Their son Rudolf Sauter (1895-1971) was also a Painter and graphic Artist, who among other things, illustrated the works of his uncle.
From the Four Winds, a collection of short stories, was Galsworthy's first published work in 1897. These and several subsequent works were published under the pen name of John Sinjohn, and it was not until The Island Pharisees (1904) that he began publishing under his own name, probably owing to the recent death of his father. His first full-length novel, Jocelyn, was published in an edition of 750 under the name of John Sinjohn – he later refused to have it republished. His first play, The Silver Box (1906), – in which the theft of a prostitute's purse by a rich 'young man of good family' is placed beside the theft of a silver cigarette case from the rich man's father's house by 'a poor devil', with very different repercussions, though justice was clearly done in each case – became a success, and he followed it up with The Man of Property (1906), the first book of a Forsyte trilogy. Although he continued writing both plays and novels, it was as a Playwright that he was mainly appreciated at the time. Along with those of other Writers of the period, such as George Bernard Shaw, his plays addressed the class system and other social issues, two of the best known being Strife (1909) and The Skin Game (1920).
Galsworthy was born at what is now known as Galsworthy House (then called Parkhurst) on Kingston Hill in Surrey, England, the son of John and Blanche Bailey (née Bartleet) Galsworthy. His family was prosperous and well established, with a large property in Kingston upon Thames that is now the site of three schools: Marymount International School, Rokeby Preparatory School, and Holy Cross Preparatory School. He attended Harrow and New College, Oxford. He took a Second in Law (Jurisprudentia) at Oxford in 1889, then trained as a barrister and was called to the bar in 1890. However, he was not keen to begin practising law and instead travelled abroad to look after the family's shipping Business. During these travels he met Joseph Conrad, then the first mate of a sailing-ship moored in the harbour of Adelaide, Australia, and the two Future novelists became close friends. In 1895 Galsworthy began an affair with Ada Nemesis Pearson Cooper (1864–1956), the wife of his cousin Major Arthur Galsworthy. After her divorce ten years later, they were married on 23 September 1905 and stayed together until his death in 1933. Before their marriage, they often stayed clandestinely in a farmhouse called Wingstone in the village of Manaton on Dartmoor, Devon. In 1908 Galsworthy took a long lease on part of the building and it was their regular second home until 1923.
Through his writings Galsworthy campaigned for a variety of causes, including prison reform, women's rights, and animal welfare, and also against censorship. During the First World War he worked in a hospital in France as an orderly, after being passed over for military Service, and in 1917 turned down a knighthood, for which he was nominated by Prime Minister David Lloyd George, on the precept that a writer's reward comes simply from writing itself. In 1921 he was elected as the first President of the PEN International literary club and was appointed to the Order of Merit in 1929. He was awarded the Nobel Prize for Literature in 1932 after he had been nominated that year by Henrik Schück, a member of the Swedish Academy. He was too ill to attend the Nobel awards ceremony and died six weeks later.
Escape was filmed in 1930 and 1948. The latter was directed by Joseph L. Mankiewicz, starring Rex Harrison, Peggy Cummins, and william Hartnell. The screenplay was by Philip Dunne.
The Skin Game was adapted and directed by Alfred Hitchcock in 1931. It starred C.V. France, Helen Haye, Jill Esmond, Edmund Gwenn, and John Longden.
One More River (a film version of Galsworthy's Over the River) was filmed by James Whale in 1934. The film starred Frank Lawton, Colin Clive (one of Whale's most frequently used actors) and Diana Wynyard, and featured Mrs. Patrick Campbell in a rare sound film appearance.
Galsworthy's short story The Apple Tree was adapted into a radio play for Orson Welles' Lady Esther Almanac radio series on CBS, first broadcast on 12 January 1942; the play was again produced by Welles for CBS on The Mercury Summer Theatre of 6 September 1946. The 1988 film A Summer Story was also based on The Apple Tree.
The NBC University Theater aired radio adaptations of his plays Justice on 31 October 1948 and The Patrician on 26 February 1950.
Galsworthy lived for the final seven years of his life at Bury in West Sussex. He died from a brain tumour at his London home, Grove Lodge, Hampstead. In accordance with his will he was cremated at Woking, with his ashes then being scattered over the South Downs from an aeroplane, but there are also memorials to him in Highgate 'New' Cemetery and in the cloisters of New College, Oxford, cut by Eric Gill. The popularity of his fiction waned quickly after his death but the hugely successful television adaptation of The Forsyte Saga in 1967 renewed interest in his work.
In 2007, Kingston University opened a new building named in recognition of his local birth. Galsworthy Road in Kingston, the location of Kingston Hospital, is also named for him.