Percy entered the war in France with the 14th Field Ambulance on 20th August 1914. He was awarded the Meritorious Service Medal for devotion to duty whilst in France, whilst attached to H.Q. VIth Corps. On 7th July 1919, he accompanied Lt/Col L T Poole who proceeded to Paris in charge of the R.A.M.C. detachment representing the IX Corps, Rhine Army in the Victory march on 14th July 1919. Percy was one of two R.A.M.C. personnel representing the 141st Field Ambulance.