He enlisted in the Australian Imperial Force on 11 April 1916 and sailed for England in May, attached to the Cyclist Training Battalion.
[1] On 30 October 1924, Mant was admitted as a solicitor by the New South Wales Supreme Court and began working for Ellison, Rich & Son.
He returned to active service for World War II on 17 March 1941, first with the Citizen Military Forces and then, from 28 July 1942, as a captain in the AIF.
In September 1942 he was promoted major and in October 1943 lieutenant colonel, becoming chief legal officer of the First Australian Army from 1942 to 1943 and of the Second from 1944 to 1945.
He was also chairman of the Wentworth Boy Scouts' Association from 1953 to 1973 and a founder of the Australian Outward Bound Memorial Foundation.