Allingham wanted to join the war effort in August 1914 as a despatch rider, but his critically ill mother managed to persuade him to stay at home and look after her. However, after his mother died in 1915, aged 42, Allingham enlisted with the Royal Naval Air Service (RNAS). He became formally rated as an Air Mechanic Second Class on 21 September 1915, and was posted to Chingford before completing his training at Sheerness, Kent. His RNAS serial number was RNAS F8317.