The ship was initially assigned to the Home Fleet, although she was temporarily deployed in the Red Sea during the Abyssinia Crisis of 1935–36.

The ship was on anti-submarine patrols during the invasion of Normandy, and was employed as a troop transport after VE Day for returning Canadian servicemen.

Cygnet carried a maximum of 473 long tons (481 t) of fuel oil that gave her a range of 5,500 nautical miles (10,200 km; 6,300 mi) at 15 knots (28 km/h; 17 mph).

Cygnet was detached from the Home Fleet during the Abyssinian Crisis, and deployed in the Red Sea from September 1935 to April 1936.

[9] On 2 July, whilst escorting the British battleship Nelson, St. Laurent received word that the unescorted British passenger ship SS Arandora Star had been torpedoed by U-47, about 125 nautical miles (232 km; 144 mi) northeast of Malin Head, Ireland.

Arriving some four and a half hours after the ocean liner sank, the ship rescued 857 survivors, including German and Italian prisoners of war.

Together with the British sloop Sandwich, she badly damaged the German submarine U-52 whilst defending Convoy HX 60 on 4 August.

[6] Whilst escorting Convoy ON 33 in November in a gale, the ship was damaged severely enough by the weather that she was forced to return to Halifax for repairs.

[6] In early December 1942, the ship's director-control tower and rangefinder were exchanged for a Type 271 target indication radar mounted above the bridge.

Whilst assigned to Escort Group C1 defending Convoy ON 154 in late December 1942, St. Laurent had her first victory; In the early hours of 27 December 1942, while north of the Azores, she sighted a U-boat on the surface which she engaged with gunfire, followed by a depth charge attack as the boat crash-dived.

On D-Day itself – 6 June 1944 – she was deployed with the Canadian destroyers Chaudière, Gatineau, Kootenay and Ottawa stationed in the entrance to the English Channel to prevent U-boat attacks on the invasion convoys.

On 8 August she was unsuccessfully attacked by a glide bomb, and on the 13th she and Ottawa rescued survivors from U-270 which had been sunk with depth charges by a Sunderland aircraft.

St. Laurent returned to service in April 1945, and was attached to the Halifax Escort Force for convoy defence off the east coast.

[6] During her service St. Laurent was credited with the destruction of one U-boat: A model of HMS Cygnet by Norman A. Ough is held by the National Maritime Museum.