 Mulock joined 13 Battery, Canadian Field Artillery in 1911.He enrolled in the Canadian Expeditionary Force in September 1914 and served as a corporal with the 1st Battery, 1st Brigade, and 12th Battery, 3rd Brigade, Canadian Field Artillery in Canada and England until December 1915.After transferring to the Royal Naval Air Service and undergoing pilot training, he was posted to 1 Naval Wing.