Holt enlisted in the Militia in February 1939, joining a part-time artillery unit for businessmen and professionals. He was given indefinite leave during his ministerial service. In May 1940, without resigning his seat, Holt enlisted in the Australian Imperial Force with the intent of becoming a full-time soldier. Several of his parliamentary colleagues did likewise at various points in the war. Holt was posted to the 2/4th Field Regiment, holding the rank of gunner. He had been offered a commission as an officer in the Royal Australian Air Force, but declined due to his lack of experience. In a press statement, Holt said "as the youngest member of the House, I could not feel happy in my position if I were not prepared to make some sacrifice and take an active part". He was sent to Puckapunyal for training, and expected to be posted to North Africa or Palestine.