At the outbreak of the First World War, Campbell initially enlisted as a motorcycle dispatch rider and fought at the Battle of Mons in August 1914. Shortly afterwards he was commissioned as a second lieutenant in the 5th Battalion, Queen's Own (Royal West Kent Regiment), a Territorial Force unit, on 2 September 1914. He was soon drafted into the Royal Flying Corps, where he served as a ferry pilot, for his instructors believed he was too clumsy to make the grade as a fighter pilot.