Nimmo was appointed as a lieutenant in the Permanent Military Forces upon graduation on 3 November 1914, having held the position of the senior cadet of his 40-strong class, known as the company sergeant major. He was commissioned as a lieutenant into the Australian Imperial Force (AIF) and joined the 5th Light Horse Regiment, part of Colonel Granville Ryrie's 2nd Light Horse Brigade which was forming from men recruited in Queensland. On 21 December the regiment sailed from Sydney for the Middle East aboard , a White Star Line ocean liner that had been converted into a troopship and redesignated HMAT A34.
Nimmo became a lieutenant in the Permanent Military Forces upon graduating on November 3, 1914, serving as the senior cadet of his 40-member class. He was commissioned into the Australian Imperial Force (AIF) and joined the 5th Light Horse Regiment, part of Colonel Granville Ryrie's 2nd Light Horse Brigade, which was being formed from recruits in Queensland. The regiment departed Sydney for the Middle East on December 21 aboard HMAT A34.