In 1897, John hit submerged rocks diving into the sea at Tenby, suffering a serious head injury; the lengthy convalescence that followed seems to have 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 Pierre Puvis de Chavannes.

The need to support Ida Nettleship (1877–1907), whom he married in 1901, led him to accept a post teaching art at the University of Liverpool.

Early in 1901, John married his first wife, Ida Nettleship (1877–1907); the couple had five children. After her death in 1907, his mistress Dorothy “Dorelia” McNeill, a Bohemian style icon, became his partner; they lived together most of the time from 1904 until his death and had two children, but never married. One of his sons (by his first wife) was the prominent British Admiral and First Sea Lord Sir Caspar John. His daughter Vivien John (1915–1994) was a notable painter.

Some time in 1910, John fell in love with the town of Martigues, in Provence, located halfway between Arles and Marseilles, and first seen from a train en route to Italy. John wrote that Provence “had been for years the goal of my dreams” and Martigues was the town for which he felt the greatest affection. “With a feeling that I was going to find what I was seeking, an anchorage at last, I returned from Marseilles, and, changing at Pas des Lanciers, took the little railway which leads to Martigues. On arriving my premonition proved correct: there was no need to seek further.” The connection with Provence continued until 1928, by which time John felt the town had lost its simple charm, and he sold his home there.

John became a leader of the New English Art Club, where he chiefly exhibited. With his vivid manner of portraiture and his ability to catch unerringly some striking and usually unfamiliar aspect of his subject, he superseded Sargent as England’s fashionable portrait painter. In 1921 he was elected an Associate of the Royal Academy and elected a full R.A. in 1928. He was named to the Order of Merit by King George VI in 1942. He was a trustee of the Tate Gallery from 1933 to 1941, and President of the Royal Society of Portrait Painters from 1948 to 1953. He was awarded the Freedom of the Town of Tenby on 30 October 1959.

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 1937 evidence a resurgence of his powers, and amounted to “the St. Martin’s summer of his creative genius”.

In later life, John wrote two volumes of autobiography, Chiaroscuro (1952) and Finishing Touches (1964). In old age, although John had ceased to be a moving force in British art, he was still greatly revered, as was demonstrated by the huge show of his work mounted by the Royal Academy in 1954. He continued to work up until his death in Fordingbridge, Hampshire in 1961, his last work being a studio mural in three parts, the left hand of which showed a Falstaffian figure of a French peasant in a yellow waistcoat playing a hurdy-gurdy while coming down a village street. It was Augustus John’s final wave goodbye.

John was, throughout his life, particularly interested in the Romani 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 John’s children by both women, travelled in a caravan, in gypsy fashion. Later on he became the President of the Gypsy Lore Society, a position he held from 1937 until his death in 1961.

He joined the Peace Pledge Union as a pacifist in the 1950s, and was a founder member of the Committee of 100. 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 died at Fordingbridge, aged 83.

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.

Augustus John and his student James Dickson Innes spent two years painting in the Arenig valley around 1910, especially the mountain Arenig Fawr. In 2011 this period was made the subject of a BBC documentary titled The Mountain That Had to Be Painted.

During World War I, John was attached to the Canadian forces as a war artist and made a number of memorable portraits of Canadian infantrymen. The end result was to have been a huge mural for Lord Beaverbrook and the sketches and cartoon for this suggest that it might have become his greatest large-scale work. However, like so many of his monumental conceptions, it was never completed. As a war artist, John was allowed to keep his facial hair and therefore, he and King George V were the only Army officers in the Allied forces to have a beard, apart from pioneer sergeants and those who were allowed unshaven for medical reasons. After two months in France he was sent home in disgrace after taking part in a brawl. Lord Beaverbrook, whose intervention saved John from a court-martial, sent him back to France where he produced studies for a proposed Canadian War Memorial picture, although the only major work to result from the experience was Fraternity. In 2011, the Duke and Duchess of Cambridge finally unveiled this mural at the Canadian War Museum in Ottawa. This unfinished painting, The Canadians Opposite Lens, is 12 feet high by 40 feet long.