Born in Montreal, Holmes moved with his family to Owen Sound in 1903. He enlisted in the 147th Infantry Battalion C.E.F. in 1915, but later transferred to the 4th Canadian Mounted Rifles. In October 1917, his unit took part in the violent opening assault on the German position at Passchendaele. During this action Private Holmes, under heavy enemy fire, captured single-handed an important "pill-box" strongpoint which had been holding up the right flank of the Canadian advance. He was awarded the Victoria Cross for his valour in this battle.