On August 4, 1910, at Portsmouth, England, Rainbow was commissioned the first unit of the infant RCN. One of a class of 20 "protected cruisers," she had served in the RN since 1893. Rainbow arrived at Esquimalt on November 7, 1910, and carried out training duties, ceremonial visits, and some fishery patrols until the end of 1912. She then lay largely idle until the outbreak of the First World War, during which, apart from two submarines, she was the sole defender of Canada's western seaboard. The German threat in the Pacific ended with the defeat of Admiral Graf von Spee's squadron at the Battle of the Falkland Islands in December, 1914, and thereafter Rainbow patrolled the Pacific coast as far south as Panama. In 1916 and early 1917 she was used in the transporting of $140 million in Russian bullion between Esquimalt and Vancouver. By 1917 her crew was needed to man patrol vessels on the east coast, and on May 8 she was paid off. Rainbow was re-commissioned on July 5 to serve as a depot ship at Esquimalt. Paid of again on June 1, 1920, she was sold to a Seattle shipbreaker.