Mortimer was the fourth son of Edward and Dora Pim of Linden, Maghull. They had already lost one son, killed in a riot in Bulawayo, southern Africa, whilst on diplomatic service. Mortimer joined the Cadet Force at Petersfield School, and then the 6th Battallion (Liverpool Rifles), King’s (Liverpool Regiment) when he returned home. He was also a founder of the first scout troop in Liverpool. He served first as a despatch rider, then an infantryman. Rifleman Pim was wounded near Flers, on the Somme, and died of his injuries. He is buried in Heilly Station cemetery, France.