Colley-Priest enlisted in the 8th Field Ambulance, part of the 8th Infantry Brigade of the Australian Imperial Force, on 19 May 1915 and embarked aboard the HMAT Ascanius on 9 November, bound for Egypt and the Western Front. While serving, he was deployed at the Somme, Racquinghem, Polygon Wood, Messines, Villers-Bretonneux and Mont Saint-Quentin. In 1917, Colley-Priest was awarded the Military Medal. The recommendation for the award noted that he displayed: