[2] Forrest originally served as a Quartermaster Sergeant with the Australian Imperial Force's 23rd Infantry Battalion.

Forrest led a daring trench raid in the Hollandscheschuur salient in September 1916; this gained him a mention in despatches and the Italian Silver Medal of Military Valor.

[4] Forrest began his string of aerial victories on 22 March 1918, when he drove down a German two-seater reconnaissance aircraft out of control, followed by the destruction of two Albatros D.V fighters over Bullecourt.

[1][6] On 2 April 1918, he and Roby Lewis Manuel destroyed a German reconnaissance aircraft southeast of Demuin.

His final tally amounted to six enemy aircraft destroyed, including the one shared with Manuel, and five more driven down out of control.

Its citation read:[8] Captain Henry Garnet Forrest (Australian Flying Corps) His leadership of patrols has been characterised by great dash and determination whether on high or low work.

He was sent home sick on HT Marathon, departing England on 6 November and arriving back in Melbourne on New Year's Day 1919.

[4] On 16 March 1920, he was one of the returned military veterans honoured by the Order of Druids at a banquet; he was granted a certificate in recognition of his service.