Fox was born in Leeds in 1891, the second of four sons to a schoolteacher mother and engineering workman father. His parents were active in the Co-operative Movement, and as a young man Fox joined the Socialist Party of Great Britain and the Industrial Workers of the World.
On the outbreak of World War I, Fox denounced it as an imperialist war in which workers had no interest. His refusal to be conscripted led to his being tried and imprisoned several times throughout the war. He was abruptly released in April 1919; later that year he authored his first book, Factory Echoes, and enrolled at Ruskin College in Oxford.
While at Oxford, Fox became the editor of New Oxford and gained a reputation as a labour journalist. He was invited to Soviet Russia in 1921 to observe the results of the recent Russian Revolution, and in 1922 he visited Dublin and established contacts with leading leftist figures there.
Following his graduation from Oxford, Fox married children's author Patricia Lynch and they spent time in London, Paris, Brussels, and Germany before eventually settling in Dublin. Fox's articles appeared in the Irish Statesman and in the late 1920s he began publishing his books through Virginia Woolf's Hogarth Press. His autobiography, Smokey Crusade, was published in 1937 and an account of his trip to Maoist China was published as China Diary in 1959.
Fox died in December 1969, three years before his wife. They are both buried in Glasnevin Cemetery.