In July 1915, Lowerson joined the Australian Imperial Force. After initial training, he embarked for the Middle East in September 1915 as a reinforcement for 21st Battalion. He arrived too late to participate in the Gallipoli Campaign and in March 1916 proceeded with his battalion to the Western Front. He fought in the Battles of the Somme, Pozières and Mouquet Farm, the latter of which saw him wounded. He was also recommended for the Military Medal for his work at Pozières but this was not awarded. After a month recovering from his wounds he rejoined his battalion and shortly afterwards was promoted to corporal. Promoted in April 1917 to acting sergeant, he was again wounded, this time during the Second Battle of Bullecourt.