Cortis served as a captain in the second contingent of the New South Wales Army Medical Corps in the Boer War, participating in operations in Orange Free State (including actions at Poplar Grove, Driefontein, Karee Siding, Vet River, Zand River, Bethlehem and Bothaville) and in the Transvaal (including actions at Johannesburg, Pretoria, Diamond Hill and Belfast). He was awarded the South African War Medal with five clasps.