On 24 July 1915, during World War I, Williams enlisted in the Australian Imperial Force (AIF). Before leaving Australia, he became engaged to Miss Doris Price, another member of the Waverley Methodist Church choir. He was sent to Europe aboard the troop transport "Argyllshire" in May 1916, having been made a corporal with the 9th Field Ambulance section. During the voyage, his vigorous delivery of ballads became popular fare with his fellow soldiers at shipboard entertainments. After military training in England, he was promoted to sergeant and sent to France with his unit in November 1916, seeing action at Armentières. In January 1917, he was transferred at General William Birdwood's request to an entertainment unit, known as the 'Anzac Coves'. He then rejoined the 9th Field Ambulance in March, saw action at Passchendaele and Messines, and was appointed regimental quartermaster-sergeant.