Champlin was launched 25 July 1942 by Bethlehem Steel Company, Fore River Shipyard, Quincy, Massachusetts; sponsored by Mrs. A. C. Brendel; and commissioned 12 September 1942.

While covering the landing and initial advances the same day, she answered the request from shore for a bombardment of the village of Camerina, so successfully that the enemy there surrendered.

She made four more Atlantic crossings on convoy escort duty from New York to North Africa and the British Isles between 21 August 1943 and 11 March 1944.

While undergoing refresher training in Casco Bay, Maine, in March 1944, Champlin was ordered out on a submarine hunt, joining an all-day operation 7 April.

Returning to Palermo, she sailed from that port 13 August for the invasion of southern France, in which she was assigned to patrol southwest of the transport area as a reserve fire support unit.

Continuing her fire support, she knocked out a bridge across the Var River near Nice upon Army request on 24 August, and a week later left the area to guard merchantmen bound for Oran.

She continued to New York, escorting a division of battleships, and began a program of training and plane guard operations which lasted through the remainder of 1944.

On 30 January, she cleared Oran to rendezvous with the group bringing President Franklin D. Roosevelt to Malta, where he was to enplane for the Yalta Conference.

Continuing to Okinawa, she arrived 12 August for local escort and patrol duty until 4 September, when she cleared on the first of two voyages to Japan in connection with occupation arrangements.