Immediately Ericsson began operations along the East Coast and Bermuda, training Naval Reserve midshipmen, exercising with submarines, making tests of her equipment and machinery, and joining in battle practice.

In the fall of 1941, she twice voyaged to Newfoundland and Iceland, escorting convoys, continuing this service after the United States entered World War II.

For the next week she offered direct fire support to the troops ashore, assisting in knocking out four enemy batteries on a ridge commanding the landing area the first day; she also screened transports lying off the beach.

She escorted convoys and carried passengers between north African and Italian ports, bombarded points near the fiercely contested Anzio area and in the Gulf of Gaeta, patrolled off anchorages and harbors, and joined in exercises preparing for the invasion of southern France.

After refresher training, Ericsson escorted a convoy to Oran from the east coast in April 1945, and while returning to Boston, on 5 May joined Atherton and Moberly in a submarine hunt off Block Island.

At Boston from 6 May to 18 June, Ericsson prepared for Pacific service, and after training in the Caribbean and at Pearl Harbor, escorted a group of transports to Saipan, arriving 13 September 1945.