Percy enlisted on 31st March 1908. He was 22 years and 2 months old and was a Valet by trade. From 1911 to 1914 he was serving in Gibraltar. He entered the war in France on 16th August 1914, as part of the team for the Assistant Director of Medical Services, 2nd Division. On 20th August 1914 he gained promotion to the rank of Corporal and served as a Clerk. He transferred to serve as part of the team of the Director of Medical Services, 1st Army on 3rd April 1915. On 16th October 1915 he gained further promotion to the rank of Serjeant. On 28th March 1916 a report by Major-General P E F Hobbs, D.A. & Q.M.G., First Army stated "The General Officer Commanding wishes to express his appreciation of the following Acts of Courage:- No 1791, Sergeant P.G. Elsey, R.A.M.C., attached Headquarters, First Army. At Aire on Wednesday, 22nd March 1916, about 5.30 pm a runaway pony, attached to a cart containing a little girl agave about 4 or 5 years, was dashing along the Rue St. Omer when Sergeant Elsey, who was walking along the street at the time, ran out and, at considerable personal risk, succeeded in bringing the animal to a standstill. This timely and plucky action possibly save the child from serious injury." On 1st January 1917 he joined the 15th Field Ambulance for duty, but on 10th November 1917 he was arrested and awaiting trial. He was court marshalled on 15th November and admitted into No 2 Military Prison, Rouen two days later. During his sentence in prison, on 11th December 1917, he was transferred to the East Lancashire Regiment. He was released from prison on 14th March 1918. On 24th April 1918 he was reported as missing, then later as a prisoner of war in Germany. He was repatriated early 1919, then transferred to Section "B" Army Reserve on 17th April 1919.