The ship, originally designated a "patrol gunboat," PG-200, was ordered by the United States Maritime Commission under a U.S. Navy contract as USS Prowse.

She was reclassified as a "patrol frigate," PF-92, on 15 April 1943 and laid down by the Walsh-Kaiser Company at Providence, Rhode Island, on 20 October 1943.

Intended for transfer to the United Kingdom, the ship was renamed Zanzibar by the British prior to her launch on 21 November 1943.

Transferred to the United Kingdom under Lend-Lease on 21 June 1944, the ship served in the Royal Navy as HMS Zanzibar (K596).

She operated out of Derry, Northern Ireland, and Greenock, Scotland, on convoy escort duty into the western Atlantic Ocean as far as Boston, Massachusetts.