Over a year after the outbreak of the First World War, which began in August 1914, Crocker enlisted into the British Army as a private soldier in the Artists Rifles, a training corps for potential officers, in November 1915. He was commissioned as a temporary second lieutenant into the Machine Gun Corps (MGC) on 26 January 1917. He had a distinguished career in the war and in April and July 1918 was awarded, respectively, the Military Cross (MC) and Distinguished Service Order (DSO). After training at the Machine Gun School at Grantham, Lincolnshire, Crocker joined the 174th Machine Gun Company, part of the 59th (2nd North Midland) Division, a Territorial Force (TF) formation then serving in the trenches of the Western Front as part of the British Expeditionary Force (BEF). He fought with his company, which in early March 1918 became part of the 59th Machine Gun Battalion, in the Battle of Passchendaele (also known as the Third Battle of Ypres) in mid-1917 and in the German Army's Spring Offensives of 1918. He was promoted to temporary rank of lieutenant on 26 July 1918.