As the Second World War loomed, Leavey joined up and served in the 5th Royal Inniskilling Dragoon Guards in France and Belgium; he only just escaped at Dunkirk, being one of the last to be evacuated. On arriving back in Britain the regiment underwent training in the midlands where Leavey organised a pack of fox hounds with which to go fox hunting. He was pleased to return to liberate France after D Day, fighting through to the end of the war. He was mentioned in despatches and reached the rank of Major. After demobilisation in 1946, Leavey was employed by the family firm of Smith and Nephew and became a Director in 1948. He joined the Yorkshire Hussars in the Territorial Army as a Major in 1952, but left in 1955.