John took his M.B. and B.Ch at Trinity College, Dublin in 1913. He received a temporary commission as Lieutenant and entered the war in France on 25th August 1914. He was wounded near Ypres. His name appeared in the wounded section of the casualty lists on 5th May 1915. He was the only son of Aquila and Maude MacMahon (nee Tweedy) of 28 Herbert Place, Dublin.