James McMahon was 38 years and three months old when he enlisted on 17 October, 1916.He was born in Clunes, Victoria, the son of Patrick McMahon (1859-1914) and his wife Hannorah, nee Savage (1854-1904). He had one brother, John Ignatius McMahon, b. 1892. By trade, he wrote that he was a labourer and Plate Layer, 5' 8" in height, with black to grey hair  and grey eyes. He was, by faith, a Roman Catholic

By the time James McMahon enlisted, both his parents were dead,  Hannorah in 1904 and Patrick McMahon in 1914. He gave as his next of kin his wife, Mary Frances Claire, nee Noonan, known as Molly. At this stage, they had one son, William (Billy) James McMahon, born 1914. Molly was living at 134 Simpson St., East Melbourne. The Electoral Rolls for 1914 show them living in Barwon, Corio,Victoria. 

Jim McMahon was placed with the 2nd Pioneer Regiment, Reinforcements 7, for training. Each Division was allocated a Pioneer Battalion composed of light military combat engineers and support soldiers located at the very forward edge of the battle area and therefore highly dangerous. Their tasks included constructiing defensive positions - trenches,  dugouts and command posts, and prpeared barbed wire defences, plus building construction and  maintenance of positions. 

Following training, probably at Seymour, on 19 October 1916 James McMahon was placed with the 7/2 Pioneers. They embarked on H.M.A.T. Hororata A20, owned by the New Zealand Shipping Co. Ltd. and leased by the Australian Government until 11 September, 1917. They landed at Plymouth, England on 29 January, 1917 and on 30 January, marched into camp at Fovant. on 12 July,  where James McMahon was reassigned, taken on strength from the 2nd Pioneer Battalion, ex-Fovant, and transferred to the 23rd Battalion, leaving from Southampton and landing at Rouen. 

James McMahon was two months with the 23rd, before coming down with Trench Fever on 28 10 1917, an illness characterised by high temperatures, headache, myalgia  and rashes on the torso, and pain in the long bones of the leg. The illness was treated by quinine. During the Great War, it is estimated that 9.7 military personnel and 10 million civilians died of trench fever, Of these deaths, an estimated 5.7 million were soldiers fighting for the Allies. James McMahon was lucky to survive, but such was his ill health, that he was returned to Australia, embarking on 15 February 1918 on the hospital ship Landstephen Castle. He was noted as suffering from premature senility. He was discharged from further service in Melbourne on 25 May, 1918.