Roberts was intrigued by Post-impressionism and Cubism, an interest fuelled by his travels in France and Italy after leaving the Slade in 1913. He joined Roger Fry's Omega Workshops in October 1913. The modest income that Omega paid, enabled him to create challenging Cubist-style paintings such as The Return of Ulysses owned by Castle Museum and Art Gallery, Nottingham.
After leaving Omega he was taken up by Wyndham Lewis, who was forming a British alternative to Futurism. Ezra Pound had suggested the name Vorticism and Roberts' work was featured in both editions of the Vorticist literary magazine BLAST.
In 1916 Roberts enlisted in the Royal Artillery as a gunner, serving on the Western Front, and the following year he was recruited by the War Office as an official war artist. His experiences at the front shifted the direction of his work, and significant pieces from his wartime output include The First German Gas Attack at Ypres, a powerful painting that dramatically depicts the horror of war.
After the war, his subject matter turned to the documentation of urban life and portraiture. In 1923 he held his first one-man exhibition at the Chenil Gallery in London and, two years later, he was appointed visiting lecturer at the Central School of Art, a post he held until 1960.
In 2005, WILLIAM ROBERTS: AN ENGLISH CUBIST by ANDREW GIBBON WILLIAMS (Lund Humphries), the standard monograph on this artist, was published.
From 1961 to 1962 Roberts painted The Vorticists at the Restaurant de la Tour Eiffel, Spring 1915, a nostalgic recollection of a boisterous Vorticist gathering in 1915. He was elected an RA of the Royal Academy in 1966, and continued to depict large scale urban scenes in his paintings until his death in 1980.