John Mahon was born in Dublin in 1901, the son of John Lincoln Mahon, a comrade of both William Morris and Frederick Engels. The family soon moved to London, where they lived at Brixton. He was educated at St. Olive's and St, Saviour's Grammar School in Tooley Street, in Southwark. His first job was at Buck and Hickman, an engineering works in the East End of London.
He joined the Communist Party in 1920 and was very soon employed as a full-time political worker until his retirement. John Mahon was on the staff of the National Minority Movement, where h worked closely with Harry Pollitt. He was Pollitt's campaign organiser in his election contest in Whitechapel and St. George's.
Mahon (centre) was then Industrial Organiser and later became District Secretary of the London District Communist Party, a member of the Executive and Political Committees. he is pictured (left) in the St Pancras election of 1950 and (right) showing off a new Daily Worker anti-Tory poster.
He died in 1975, shortly after completing his massive biography of Harry Pollitt.