Will Arnold-Forster was the youngest son of Hugh Oakeley Arnold-Forster, a Liberal Unionist MP and his wife, Mary Story-Maskeline. He inherited an interest in art from his mother, and studied at the Slade School between 1905 and 1908 where he won several prizes. He moved to Italy in 1911 living in Tuscany. At the outbreak of war he joined the Royal Navy.
After the war, he married Katharine Laird Cox (known as Ka), who was then working at the Admiralty, and they moved to Cornwall where they purchased 'The Eagle’s Nest'. He was an enthusiastic gardener, and his garden at 'The Eagle’s Nest' was described as spectacular. He worked on the Memorial Garden at St Ives, and with the sculptor Barbara Hepworth on her garden there.
As a Labour politician, Arnold-Forster was a strong human rights advocate, and became involved in the creation of the League of Nations (1920). In the interwar period he was influential in foreign policy debates that tried to find an alternative to war and argued for multilateral disarmament. During the Second World War he continued to advance ideas for a new international body with more coercive powers. After the war he continued writing and speaking on internationalism and the United Nations.
As an artist, he first joined the St Ives Arts Club in 1909 and was noted for landscapes and pastels. His work is included in the National Portrait Gallery, London.
Will and Ka were interested in progressive education, and they sent their son Mark, aged seven, to boarding school in Switzerland, and two years later to a boarding school in Salem, Baden-Württemberg run by Kurt Hahn. Hahn, a Jew, was imprisoned in Germany, but was released with the assistance of the Arnold-Forsters and fled to Scotland in 1933. Together they were instrumental in the founding of Gordonstoun. Will was the first chairman of the board of directors and Mark was one of the first pupils.
Ka died suddenly in 1938 at the age of 51, while her husband was in North America on a peace mission. The following year he married his friend Ruth Leigh-Mallory (widow of George Mallory). She died three years later of cancer.