Shortly before World War I, Mather's family emigrated to Australia, where he received his education. He studied medicine, the family profession at Sydney University. Finding himself in England without prospects at the height of the Great Depression, he enlisted in the Royal Horse Artillery, but failed to gain a commission. He therefore applied to join the Indian Army, in which he rose through the ranks, becoming a sergeant at the outbreak of World War II in 1939. He served in the Iraq campaign under Slim, and ended the war as an acting lieutenant-colonel. After India gained independence in 1947, he rejoined the British Army, serving in the Royal Artillery until he retired in 1959.