Built for the Royal Navy as the Upholder-class submarine HMS Unicorn (pennant number S43) she was purchased by Canada when the United Kingdom decided to move to an all-nuclear power fleet.

They are equipped with two Paxman Valenta 1600 RPS SZ diesel engines each driving a 1.4-megawatt (1,900 hp) GEC electric alternator with two 120-cell chloride batteries.

In British service, the submarines were equipped with 14 Tigerfish Mk 24 Mod 2 torpedoes and four UGM-84 Sub-Harpoon missiles.

[4] In 2014, the Government of Canada purchased 12 upgrade kits that will allow the submarines to fire the Mk 48 Mod 7AT torpedoes.

[9] She was the last ship built at Cammell Laird until construction began on HMS Queen Elizabeth in June 2010.

[9] Looking to discontinue the operation of diesel-electric boats, the British government offered to sell Unicorn and her sister submarines to Canada in 1993.

[11] The submarine was commissioned into Maritime Command as HMCS Windsor with the hull number SSK 877 on 4 October 2003.

[12] In April 2002, after departing for the submarine's first training mission, Windsor was forced to return to port after the discovery of a faulty seal in the communications mast.

These included bad welds in the hull, broken torpedo tubes, a faulty rudder and tiles on the side of the sub that continually fell off.

[12] The work was expanded in June to incorporate other maintenance needs on the submarine as well as a main sonar system upgrade for a total cost of $18 million.

[21] In Fall 2015, Windsor deployed for large NATO naval exercises Joint Warrior and Trident Venture with Athabaskan, Halifax, Montréal, Goose Bay and Summerside, returning on 17 December.

[27] In September Windsor was among the Canadian warships deployed to the NATO naval training exercise "Cutlass Fury" off the east coast of North America.

[28] Beginning in February 2017, Windsor deployed into the Atlantic Ocean, taking part in a NATO naval exercise and monitoring sea traffic in shipping lanes.

Although scheduled to participate in a 14-day multi-national exercise, Windsor was compelled to return to Halifax where repairs were expected to take four weeks.