[3] Commissioned in November 1915, Castor was the flagship of Commodore (D), assigned to the 11th Destroyer Flotilla in the Grand Fleet.

In April 1920, she recommissioned at Chatham Dockyard for service in the 2nd Light Cruiser Squadron in the Atlantic Fleet.

[3] In 1921, Castor joined the light cruisers HMS Caledon, Cordelia, and Curacoa and the destroyers Vanquisher, Vectis, Venetia, Viceroy, Violent, Viscount, Winchelsea, and Wolfhound in a Baltic Sea cruise, departing the United Kingdom on 31 August 1921.

The ships crossed the North Sea and transited the Kaiser Wilhelm Canal to enter the Baltic, where they called at Danzig in the Free City of Danzig; Memel in the Klaipėda Region; Liepāja, Latvia; Riga, Latvia; Tallinn, Estonia; Helsinki, Finland; Stockholm, Sweden; Copenhagen, Denmark; Gothenburg, Sweden; and Kristiania, Norway, before crossing the North Sea and ending the voyage at Port Edgar, Scotland, on 15 October 1921.

[3] Castor underwent a refit from November 1925 to September 1926, then began transporting troops to China in October 1927.