Geoffrey Selwyn Walker was born in Ballarat, Victoria, was single and unemployed when he enlisted with the Infantry Brigade Train 8 on 29 September 1915 at the age of 18. He had previously served in the 71st Infantry, Citizen Military Forces. His unit embarked from Melbourne, Victoria, on board HMAT A41 Bakara on 5 November 1915. He was appointed to the rank of Lance Corporal on 9 March 1916 before transferring to the 5th Divisional A.A.O.C. and receiving a promotion to Corporal in early 1917.

On 14 October 1917 he joined the Australian Flying Cops as a Cadet and, having graduated, received a commission to the rank of 2nd Lieutenant on 23 April 1918. He returned to Australia on 15 August 1918.
Tree No. 510