"Augustus was celebrated first for his brilliant figure drawings, and then for a new technique of oil sketching. His work was favourably compared in London with that of Gauguin and Matisse. He then developed a style of portraiture that was imaginative and often extravagant, catching an instantaneous attitude in his subjects."
In the summer of 1897, John was seriously injured while swimming, and the lengthy convalescence that followed seems to have actually stimulated his adventurous spirit and accelerated his artistic growth. In 1898, he won the Slade Prize with Moses and the Brazen Serpent. John afterward studied independently in Paris where he seems to have been influenced by Puvis de Chavannes.
He was, throughout his life, particularly interested in the Roma people (whom he referred to as Gypsies), and sought them out on his frequent travels around the United Kingdom and Europe. For a time, shortly after his marriage, he and his family, which included his wife Ida, mistress Dorothy (Dorelia) McNeill, and Johns' children by both women, travelled in a caravan, in gypsy fashion.
Although well-known early in the century for his drawings and etchings, the bulk of John's later work consisted of portraits, some of the best of which were of his two wives and his children. He was known for the psychological insight of his portraits, many of which were considered "cruel" for the truth of the depiction. Lord Leverhulme was so upset with his portrait that he cut out the head (since only that part of the image could easily be hidden in his vault) but when the remainder of the picture was returned by error to John there was an international outcry over the desecration.
By the 1920s John was Britain's leading portrait painter. John painted many distinguished contemporaries, including T. E. Lawrence, Thomas Hardy, W. B. Yeats, Lady Gregory, Tallulah Bankhead, George Bernard Shaw, the cellist Guilhermina Suggia, the Marchesa Casati and Elizabeth Bibesco. Perhaps his most famous portrait is of his fellow-countryman, Dylan Thomas.
It was said that after the war his powers diminished as his bravura technique became sketchier. One critic has claimed that "the painterly brilliance of his early work degenerated into flashiness and bombast, and the second half of his long career added little to his achievement." However, from time to time his inspiration returned, as it did on a trip to Jamaica in 1937. The works done in Jamaica between March and May of 1937 evidence a resurgence of his powers, and amounted to "the St. Martin's summer of his creative genius".
Of his method for painting portraits John explained:
Make a puddle of paint on your palette consisting of the predominant colour of your model's face and ranging from dark to light. Having sketched the features, being most careful of the proportions, apply a skin of paint from your preparation, only varying the mixture with enough red for the lips and cheeks and grey for the eyeballs. The latter will need touches of white and probably some blue, black, brown, or green. If you stick to your puddle (assuming that it was correctly prepared), your portrait should be finished in an hour or so, ''and be ready for obliteration before the paint dries, when you start afresh.
Early in 1900, he married his first wife, Ida Nettleship (1877 – 1907), with whom he had five children. After her death in 1907, his mistress Dorothy "Dorelia" McNeill took her place and later became his second wife, with whom he had two children. One of his sons (by his first wife) was the prominent British Admiral and First Sea Lord Sir Caspar John. By Ian Fleming's widowed mother, Evelyn Ste Croix Fleming née Rose, he had a daughter, Amaryllis Fleming (1925 – 1999), who became a noted cellist. Another of his sons is the television director Tristan de Vere Cole noted for his contributions to TV series from the Sixties to the Eighties. His son Romilly (1906-1986) was in the RAF, briefly a civil servant, then a poet, author and an amateur physicist.
He joined the Peace Pledge Union as a pacifist in the 1950s, and on 17 September 1961, just over a month before his death, he joined the Committee of 100's anti-nuclear weapons demonstration in Trafalgar Square, London. At the time, his son, Admiral Sir Caspar John was First Sea Lord and Chief of Naval Staff.
He is said to have been the model for the bohemian painter depicted in Joyce Cary's novel The Horse's Mouth, which was later made into a 1958 film of the same name with Alec Guinness in the lead role.
Michael Holroyd published a biography of John in 1975 and it is a mark of the public's continued interest in the painter that Holroyd published a new version of the biography in 1996.