At the outbreak of the First World War, Rainbow was the only major Canadian or British warship on the western coast of North America.

[1] Rainbow's main armament was two single-mounted QF 6-inch (152 mm) guns placed along the centreline of the forecastle and poop deck.

The cruisers were also fitted with four 14 in (360 mm) torpedo tubes, of which three were installed on the upper deck, two broadside abreast the mainmast and one in the bow.

On 17 December 1901 she was commissioned at Devonport by Captain Thomas Young Greet for service in the cruiser squadron as an additional ship in home waters.

[3] She arrived back at Devonport from a tour of the Mediterranean with the squadron in April 1902,[4] and took part in the fleet review held at Spithead on 16 August 1902 for the coronation of King Edward VII.

[5] Captain Charles Delabere Granville was appointed in command on 20 August 1902,[6] and visited Souda Bay, Crete, with other ships of the squadron for combined manoeuvres the following month.

[9] During the following years, she saw a severe reduction in fleet support due to her high operating cost, resulting in only minor modernization.

She and HMS Niobe were purchased from the Admiralty to be used as training ships at Royal Naval College of Canada in Halifax, Nova Scotia.

This required new heating systems, an up-to-date galley, the latest in Marconi wireless, the enlargement of the cadet gunroom and principal messes and the removal of the obsolete secondary armament.

[11] After commissioning, Rainbow was assigned to the west coast of Canada and was the first Canadian ship to sail around South America by the Strait of Magellan.

After some discussion with the passengers, who had taken over the vessel, those aboard Komagata Maru agreed to leave Vancouver only when supplies for the ship were provided.

[24] The vessel remained the only source of protection for shipping in western North America until the arrival of the Japanese armoured cruiser Izumo.

[27] In early 1916, Rainbow was still patrolling the west coast of North America, performing reconnaissance on German shipping.

[29] In 1916 and early 1917, Rainbow was used to transport $140,000,000 in Russian gold bullion (valued in 1917 Canadian dollars), between Esquimalt and Vancouver.