Reid began his military career in the Officer Training Corps prior to the First World War. He was commissioned a temporary second lieutenant in the Argyll and Sutherland Highlanders on 15 August 1914, in the early days of the war. However, it was with the Black Watch that he went into action; in January 1915, he was wounded at the Battle of Festubert while with the Watch. The wound could not have been serious, as Reid began pilot's training that same month. The following month, on 16 February, he was confirmed as a Second Lieutenant. He earned Aviators' Certificate No. 1900, awarded on 16 October 1915. Upon completion of training, he was assigned to No. 25 Squadron RFC; his appointment as a flying officer dated from 15 November 1915. Reid destroyed enemy aircraft on 16, 19, and 21 May 1916, using a Royal Aircraft Factory FE.2b with Lieutenant James Anderson Mann as his pilot. Reid won a Military Cross (MC) on 24 June 1916 as a result of these victories.