The Flower class were initially designed for coastal service during the Second World War, but due to the demands of the Battle of the Atlantic, Agassiz was used primarily as an ocean escort for convoys crossing the Atlantic Ocean in engagements with German submarines.

[4] Based on a traditional whaler-type design, the initial Canadian ships of the Flower class had a standard displacement of 950 long tons (970 t).

[3] The Royal Canadian Navy initially ordered 54 corvettes in 1940 and these were fitted with Mark II Oropesa minesweeping gear used for destroying contact mines.

Corvettes were first fitted with basic SW-1 and SW-2 CQ surface warning radar, notable for their fishbone-like antenna and reputation for failure in poor weather or in the dark.

[11] Most Canadian Flower-class corvettes had their forecastles extended which improved crew accommodation and seakeeping.

This allowed for the installation of Oerlikon 20 mm (0.8 in) cannon, replacing the Browning and Vickers machine guns.

[12] The vessel was ordered as part of the 1939–1940 Progamme in January 1940,[14] and laid down on 29 April 1940 by Burrard Dry Dock Co. Ltd. in North Vancouver, British Columbia.

[13] In August 1941, Agassiz was part of the escort 57-ship HX 143 convoy, which was guided around a German U-boat concentration in the North Atlantic.

[18] Agassiz participated in the battle for convoy ON 102 with Mid-Ocean Escort Force (MOEF) group A3 in July 1942.

[13] During the battle for ON 115, Agassiz opened fire on a surfaced U-boat, then aided in the recovery of the crew of the sunken RMS Lochkatrine.

The corvette then took the tanker G. S. Waldron under tow after the ship had been torpedoed and crippled and brought it to safety.

The corvette was part of the escort of the convoy TAW 12, travelling between Trinidad, Aruba and Key West that lost several ships.

[6] The ship's first major refit took place at Liverpool, Nova Scotia from early January 1943 until mid-March.

[13] For service in during the Second World War, Agassiz was awarded the battle honours "Atlantic 1941–45"[21] and "Gulf of St. Lawrence 1944.