Duncan McCallum Allan enlisted 1 December 1915, in Queen's University 46th Battery Canadian Forces Artillery (C.F.A.), and was assigned No. 304576. During his time in the C.E.F., he served in the 46th Battery C.F.A., at Headquarters, 3rd Canadian Divisional Artillery, in the 3rd Canadian Divisional Signal Company, the 5th Battery C.F.A., and the 3rd Canadian Division Ammunition Column. Duncan Allan was discharged honourably, at Toronto, Ontario, Canada, on 30 March 1919. He was awarded the War Service Badge Class "A" No. 155941.
Duncan McCallum Allan stated that after the War "he could not imagine sitting in a lecture hall where one had to think; he had turned his head off." He joined a lumber company that sent him to the West Coast of Canada for approximately five years, where he met his future wife. Business was not his forte. He returned to studies, to graduate from the Palmer School of Chiropractic in Pittsburg, Pennsylvania, and then from the University of Pittsburg in science. This led to a position at the University of Toronto, in research, but when government funding to universities was decreased during the Great Depression of the 1930s, he was without a job. To make a living, he did various things and eventually taught Histology at the Canadian Memorial Chiropractic College in Toronto, Ontario.