Sidney enlisted into the Oxfordshire and Buckinghamshire Light Infantry on 9th November 1914. He was 20 years and 11 months old, and was a Butcher's Assistant by trade. He entered the war in France on 18th September 1915. On 18th November 1915 he embarked at Marseilles, then headed to Salonika. From 25th May 1917 he had a few admissions in a medical unit them on 7th December 1917 was invalided back to the UK. After recovering, on 13th February 1918, he was transferred to serve with the R.A.M.C. Serving some of his training with No 6 Coy at Cosham, he was then posted to the R.A.M.C. Depot at Blackpool. On 6th November 1918 he embarked at Southampton as a member of reinforcements for the British Expeditionary Force. He disembarked at Havre the next day, then on 11th November 1918 joined the R.A.M.C. Depot at Etaples, who posted him to serve with the 141st Field Ambulance, joining them in the field on 16th November 1918. On 15th January 1919 he proceeded to England for Demobilization. He transferred to Class Z Army Reserve on 23rd February 1919.