Born 9 August 1866 at Shenfield, Essex, the son of Matthew Warton Johnson, merchant, and Jessie Bridges, his wife. He was educated at Tonbridge School and St Bartholomew's Hospital, where he served as house physician. Johnson spent his working life as a medical missionary in the service of the Church Missionary Society. He was for many years at Baghdad and Nablus, and when he retired was senior medical officer at the CMS Hospital, Baghdad. He married twice: (1) in 1898, Jessie Paterson; there were two daughters of this marriage; (2) in 1905, Florence Elizabeth Neale, who survived him, but their only daughter had died in infancy. Johnson lived after retirement at Dar-es-Salaam, Croft Road, Crowborough, Sussex, where he died on 5 March 1946, aged 79.