White then taught at Stowe School, Buckinghamshire, for four years. In 1936 he published England Have My Bones, a well-received memoir about a year spent in England. The same year, he left Stowe and lived in a workman's cottage, where he wrote and "revert[ed] to a feral state", engaging in falconry, hunting, and fishing. White also became interested in aviation, partly to conquer his fear of heights. White wrote to a friend that in autumn 1937, "I got desperate among my books and picked [Malory] up in lack of anything else. Then I was thrilled and astonished to find that (a) The thing was a perfect tragedy, with a beginning, a middle and an end implicit in the beginning and (b) the characters were real people with recognisable reactions which could be forecast[...] Anyway, I somehow started writing a book." The novel, which White described as "a preface to Malory", was titled The Sword in the Stone and told the story of the boyhood of King Arthur. White was also influenced by Freudian psychology and his lifelong involvement in natural history. The Sword in the Stone was well-reviewed and was a Book of the Month Club selection in 1939.
In February 1939, White moved to Doolistown, Ireland, where he lived out the international crisis and the Second World War itself as a de facto conscientious objector. It was in Ireland that he wrote most of what would later become The Once and Future King; two sequels to The Sword and the Stone were published during this time: The Witch in the Wood (later retitled The Queen of Air and Darkness) in 1939, and The Ill-Made Knight in 1940. The version of The Sword in the Stone included in The Once and Future King differs in several respects from the earlier version. It is darker, and some critics prefer the earlier version. White's indirect experience of the war had a profound effect on these tales of King Arthur, which include commentaries on war and human nature in the form of a heroic narrative.
In 1946, White settled in Alderney, one of the smaller Channel Islands, where he lived for the rest of his life. The same year, White published Mistress Masham's Repose, a children's book in which a young girl discovers a group of Lilliputians (the tiny people in Swift's Gulliver's Travels) living near her house. In 1947, he published The Elephant and the Kangaroo, in which a repetition of Noah's Flood occurs in Ireland. In the early 1950s White published two non-fiction books: The Age of Scandal (1950), a collection of essays about 18th-century England, and The Goshawk (1951), an account of White's attempt to train a hawk in the traditional art of falconry. In 1954 White translated and edited The Book of Beasts, an English translation of a medieval bestiary originally written in Latin.
In 1958 White completed the fourth book of The Once and Future King sequence, The Candle in the Wind, though it was first published with the other three parts and has never been published separately. The Broadway musical Camelot was based on The Once and Future King, as was the animated film The Sword in the Stone.
He died on 17 January 1964 aboard ship in Piraeus, Greece (Athens, Greece) of a heart ailment, en route to Alderney from a lecture tour in the United States.
In 1977 The Book of Merlyn, a conclusion to The Once and Future King, was published posthumously.