On 24 October 1914, two months after the outbreak of war, Sing enlisted as a trooper in the Australian 5th Light Horse Regiment of the Australian Imperial Force. His Certificate of Medical Examination at the time showed that he stood at 5' 5" (165 cm) and weighed 141 lb. (64 kg). According to John Laws and Christopher Stewart, he was accepted into the army only after a recruitment officer chose to disregard the fact that Sing was part Chinese; at the time, only those of European ancestry were generally considered suitable for Australian military service.