Following shakedown training in Lake Michigan Hardhead entered a floating dry dock at Lockport, Illinois, and was towed to New Orleans, Louisiana, where she arrived on 16 May 1944.

Early 18 August she detected the Japanese cruiser Natori east of San Bernardino Strait, and closed for a surface attack.

Operating in a coordinated group with Growler and Hake, the submarine sighted a large cargo ship with escorts 8 November.

American submarines had by this time reduced Japanese merchant activity to a trickle, effectively destroying the island nation's lifeline to the outside world.

She severely damaged a freighter with her deck guns 22 June, and next day sank four coastal defense craft during an attack on Ambat Roads with Bullhead including submarine chasers Cha-42 (05°44′S 114°16′E / 5.733°S 114.267°E / -5.733; 114.267) and Cha-113 (ex-Dutch minesweeper A) (05°45′S 114°16′E / 5.750°S 114.267°E / -5.750; 114.267) and shuttle boat No.

Following her GUPPY IIA conversion, including streamlining, installation of a snorkel breathing apparatus, and larger storage batteries, the submarine recommissioned 24 March 1953.

She joined the Atlantic Fleet for training exercises and tactical drills in the years that followed, operating mainly in the Caribbean and off the East Coast of the United States.

In July 1958 Hardhead, joined Submarine Development Group 2, turning her attention from fleet operations to research and testing of equipment and tactical doctrine.

Hardhead was decommissioned, struck from the Naval Register, and transferred (sold) under the terms of the Security Assistance Program to Greece.

On 26 July 1972 raised the Greek flag at New London (Connecticut) by Lieutenant Commander Nikolaos Vassiliou and sailed to Greece on 29 December 1972.