On the outbreak of the First World War in August 1914, Howell attempted to enlist in the newly raised Australian Imperial Force but was initially rejected. The following year, he resigned his commission in the Citizens Military Force and re-applied to join the Australian Imperial Force for active service in the war; he was accepted on 1 January 1916. Due to his age, Howell was ineligible for a commission in the force and was instead granted the rank of private. Allotted to the 16th Reinforcements of the 14th Battalion, he embarked aboard HMAT "Anchises" in Melbourne, Victoria, on 14 March. Arriving in France, he was posted to the newly raised 46th Battalion on 20 May and promoted to corporal four days later. While the unit was engaged in action along the Somme, Howell was temporarily raised to sergeant in July, before taking part in the Battle of Pozières the following month. He relinquished his temporary appointments and reverted to lance corporal in August. Considered an expert shot, Howell had been trained as a sniper during his service with the 46th Battalion.