She began escorting supply convoys in June to Tobruk, Libya, until the ship was badly damaged in a collision in November.

The ship was refitted as an escort destroyer from November to April 1943 and transferred to the Royal Canadian Navy that same month as HMCS Kootenay.

The ship was given a lengthy refit in Canada from October to February 1945 and returned to the English Channel in April to protect against any last-gasp efforts by the Kriegsmarine to interfere with Allied supply lines to the Continent.

Decoy carried a maximum of 473 long tons (481 t) of fuel oil that gave her a range of 5,870 nautical miles (10,870 km; 6,760 mi) at 15 knots (28 km/h; 17 mph).

[3] Decoy was ordered on 2 February 1931 under the 1930 Naval Estimates, and was laid down at John I. Thornycroft & Company's yard at Woolston, Southampton, on 25 June 1931.

[4] The ship was refitted at Devonport Dockyard between 3 September and 20 October 1934 for service on the China Station with the 8th (later the 21st) Destroyer Flotilla and arrived there in January 1935.

In August 1938 she sailed for Qingdao, carrying representatives to apologise for incidents where drunken sailors had insulted the Japanese flag.

In December the ship began an extensive refit to repair corrosion problems, fix her boiler feedwater pumps, and replace her funnels.

After completing her repairs in January 1940, she was transferred to Freetown, joining the 20th Destroyer Flotilla, to escort convoys off the West African coast.

[5] Together with her sister Defender, she escorted Convoy US-2 carrying Australian and New Zealand troops to the Middle East through the Red Sea from 12 to 17 May.

[13] While returning from Operation Hats, Decoy, Ilex and the light cruisers HMS Orion and HMAS Sydney bombarded Scarpanto during the night of 3/4 September.

On 6 November, Decoy, together with the destroyers Defender, Hasty, Havock, Hereward, Hero, Hyperion, Ilex, Janus, Jervis, Mohawk, and Nubian screened the capital ships of the Mediterranean Fleet, which provided distant cover for the passage of Convoy MW3 from Egypt to Malta and Convoy ME3 from Malta as part of Operation MB8.

[16] On 25 February, she participated in Operation Abstention; together with Hereward and the gunboat Ladybird, Decoy landed commandos on the island of Kastelorizo, but they were overwhelmed by an Italian counter-attack.

After returning to Alexandria, she was transferred to the Eastern Fleet in the Indian Ocean where she escorted the slow ships of Force B when the Japanese carriers attacked in March–April 1942.

Before the war's end this was supplemented when her director-control tower and rangefinder above the bridge was replaced by a Type 271 target indication radar.

[16] Kootenay was extensively refitted between 2 October 1944 and 27 February 1945 at Shelburne Naval Dockyard and resumed anti-submarine patrols in the Channel in April after working up.