After the First World War broke out in August 1914, "Canopus" was commissioned on 7 August 1914, for service in the 8th Battle Squadron in the Channel Fleet, under the command of Captain Heathcoat Grant. She was detached from that duty on 21 August 1914 to operate from the Cape Verde-Canary Islands Station to support the cruiser squadron there. In early September, her sister ship relieved her and "Canopus" transferred to the South America Station to become guard ship there and provide support to the cruiser squadron of Rear Admiral Christopher Cradock. While en route, "Canopus" nearly encountered the German light cruiser off Brazil, but the German ship intercepted British radio signals before "Canopus" could locate her.