In 1916 with the beginning of World War I, at the age of 17, Lang left North Dakota to enlist in the Canadian Army at Milwaukee, Wisconsin. After service in England and France in the Canadian Engineers, Lang transferred to the Canadian Black Watch Infantry. He was seconded to the Royal Highland Regiment (the British Black Watch), with whom he participated in the Third Battle of Ypres in 1917 during World War I. He was awarded the British Distinguished Service Medal (DSM) for his actions in that engagement.