Prior to 7 December 1941, "Plunkett" operated in the Western Atlantic and in the Gulf of Mexico–Caribbean area on Neutrality Patrol. Initially in the latter area, she joined other Neutrality Patrol vessels off Tampico to prevent the departure of several German steamers, then cruised off Martinique, French Antilles to prevent the dispatching of warships, equipment, and gold to the Vichy government. Patrol and convoy missions in the North Atlantic followed, and, on 7 December 1941, she was en route from Reykjavík to Argentia.