In March 1915, Campney enlisted in the Canadian Army with No.5 Stationary Hospital (Queen's). During World War I he was deployed overseas to Egypt as part of the Dardanelles Campaign. The unit was later transferred to the Western Front where it supported Allied forces at the Battle of the Somme. In 1917, Campney was commissioned into the infantry, joining the 19th Canadian Infantry Battalion after the Battle of Vimy Ridge. He served with the unit until the Battle of Passchendaele when he was invalided to England. In England, Campney transferred to the Royal Flying Corps and remained with the unit until the Armistice.