The vessel is named after Scottish explorer Sir William Alexander, 1st Earl of Stirling, who was an early colonizer of Nova Scotia.

Designed as a light icebreaker and buoy tender, Sir William Alexander and sister ship CCGS Edward Cornwallis differ from the rest of the class by having one less deck in the superstructure and their buoy-handling derricks mounted forward.

Capable of carrying 784.0 long tons (796.6 t) of diesel fuel, Sir William Alexander has a maximum range of 6,500 nautical miles (12,000 km) at a cruising speed of 13.7 knots (25.4 km/h) and can stay at sea for up to 120 days.

[2][3] On 6 September 2005, CCGS Sir William Alexander left Halifax Harbour, together with the Canadian warships Athabaskan, Ville de Québec and Toronto,[6] to participate in a humanitarian aid mission named Operation Unison, which provided relief to part of the devastated Gulf of Mexico coast of the United States following Hurricane Katrina.

[8] On 19 September 2005, it was announced that the three warships were no longer needed in the Gulf of Mexico, given the massive U.S. military response as well as increasing civilian aid flowing into the region.

[10] On 30 September 2014, a Canadian Armed Forces Bell CH-146 Griffon helicopter clipped Sir William Alexander's antenna during a training exercise in Mahone Bay.

[11] In February 2017, Sir William Alexander was dispatched to aid the merchant vessel Thorco Crown, which had caught fire in the Cabot Strait.