Denis enlisted on 23rd June 1902. He was 19 years old, and was a School Teacher by trade. From 10th July 1902 to 22nd January 1908 he served in South Africa. He then served in the UK. On 15th August 1914 he entered the war in France as part of the Regimental Medical Officer's team for the 1st Bn. Scots Guards. He was appointed acting Corporal on 23rd November 1914.