 Mercer's greatest passion however was reserved for the Canadian militia, which he joined as a student in 1881. Posted to The Queen's Own Rifles of Canada as a private soldier, Mercer devoted much time and energy to the unit and became an excellent soldier and first-rate shot. Mercer rose steadily through the ranks of the militia, being made an officer in 1885 and a captain in 1891. In 1903 as a brevet major, Mercer led a company to Sault Ste. Marie to calm a riot by striking dock workers, his only deployment before 1914. In 1911, Mercer was promoted to Lieutenant-Colonel Commandant of the regiment and in 1913, Mercer was aide-de-camp to Defence Minister Sam Hughes during a military tour of Europe, during which he inspected the German army first-hand and became convinced that war was imminent.
