Morrison served in the Canadian Militia for many years, joining the 4th Field Battery in Hamilton in May 1897, and the 2nd Field Battery in Ottawa in 1898. In 1899 he volunteered for service with 'D' Battery, Royal Canadian Artillery in South Africa during the Second Boer War, where he fought at Belfast and Leliefontein, receiving the Distinguished Service Order for bravery at the latter battle. He published a book With the Guns in South Africa, a memoir of his service and the service of the Canadian Artillery during the Boer War.