John Joseph Chapman was born at Ballarat, Victoria, in 1889 to John Joseph and Isabella Cargill Chapman. He was a clerk by trade before joining the Australian Imperial Force. He signed up at Brisbane on 15 September 1914 and was assigned to the 9th Infantry Battalion with the rank of private. Chapman departed Brisbane aboard HMAT Omrah on 24 September 1914.
Whilst serving at Gallipoli in 1915 he was wounded and evacuated to Malta. Chapman later re-joined the 9th Battalion on the Western Front where he was wounded again during 1916. He was promoted to the rank of lieutenant on 18 June 1918.
Chapman remained in England at the end of the First World War and married Emily Alice Myatt, a furrier, on 6 September 1919. John Chapman was discharged from the Australian Imperial Force on 23 May 1920.