She arrived at Pearl Harbor on 25 May and conducted training in Hawaiian waters prior to getting underway on 11 June for the western Pacific.

En route to the Mariana Islands, Thornback conducted an average of four training dives daily, in conjunction with battle problems, drills, and emergency surfacing exercises, before she arrived at Guam on 25 June.

As lead ship of a wolf pack nicknamed "Abe's Abolishers", Thornback stood out to sea on 30 June, bound for the Japanese home islands.

By this point in the war, American and British task forces steamed within easy gun range of Japanese coastal targets with near impunity.

Six days later, a minor fire in the pump room caused a temporary shutdown in the number one air conditioning plant before swift repairs enabled the ship to continue as before.

The submarine's commanding officer noted somewhat humorously, "This area should be about as heavily traveled as the Sahara Desert after the working over it just had...." His assessment was correct—only straggling merchantmen and small patrol craft hugged the barren coasts.

Carefully maneuvering into position, Thornback fired one shot from her stern tubes at 04:29 and soon heard a small explosion which stopped the enemy's screws.

After securing from battle stations, Thornback passed through an oil slick and noted a mast from the heavily hit patrol craft.

Later on 31 July, Thornback rendezvoused with Sea Poacher off Kesennuma and proceeded north to pick up submarine Angler (SS-240) en route to a projected shore bombardment mission against Hokkaidō.

The sight of the three submarines cruising on the surface moved Thornback’s commander to write: "On this clear and sunny day, the three ships in perfect column on a flat sea made a beautiful picture tearing along at 18 knots."

Twelve minutes later, slowing to 10 knots (19 km/h), Thornback and her consorts opened fire with 5-inch (130 mm) and 40 millimeter batteries at a range of 4,200 yards (3,800 m).

Subsequently, brought to the Portsmouth Naval Shipyard, she was converted and reactivated under the Greater Underwater Propulsive Power Program (GUPPY).

With SubRon 4, the ship was based at Key West, Florida, and visited ports in the Caribbean Sea before entering the Charleston Naval Shipyard in February 1956 for overhaul.

Upon completion of this work, the submarine was deployed to the Mediterranean Sea for a tour with the Sixth Fleet before returning to Key West in March 1957.

On 2 June 1958, Thornback departed the Caribbean, bound for Londonderry Port, Northern Ireland, and operations with the joint Royal Navy-Royal Air Force antisubmarine school.

Placed in a reduced manning status on 14 April 1971, the ship was turned over to the Turkish Navy on 1 July 1971 and renamed TCG Uluçalireis (S 338) for the Ottoman admiral Uluç Ali Reis.