On 15 August 1914 Forsyth joined the Australian Imperial Force as a temporary colonel. Appointed to command the 1st Division's mounted regiment, the 4th Light Horse Regiment, he also took on responsibility for raising the 1st Light Horse Brigade, whose commander designate, Colonel Harry Chauvel, was still on duty in the United Kingdom. Forsyth embarked with the brigade on 21 October 1914 and handed over command to Chauvel on the latter's arrival in Egypt in November 1914. Forsyth then concentrated on training his own regiment, who referred to him as "Dad".