When Dennis enlisted at Warialda on 12 January 1916 he stated that he was born at Maitland, was a forty four year old Labourer, and that his mother was Mrs. Margaret O’Neill of Murrurrundi NSW. He was one of twelve Warialda district men who joined with the Inverell volunteers known as the First Contingent of The Kurrajongs on their journey to the Narrabri and Armidale Camps.
He became a member of the 33rd Battalion Headquarters and trained at Armidale and Rutherford prior to sailing from Australia on the ship HMAT A74 Marathon in May 1916. After nine weeks at sea the Battalion arrived in England where further training was undertaken at Salisbury Plain before they were sent to France in November. Dennis served until he became ill in February 1918 and was admitted to hospital. After transfer to England in April the decision was made to return him to Australia due to his health. He was medically discharged in July 1918.