Flasher arrived at Pearl Harbor from New London 15 December 1943 to prepare for her first war patrol, for which she sailed 6 January 1944.

Bound for her patrol area off Mindoro, she sank her first target 18 January, sending a 2,900-ton former gunboat Yoshida Maru to the bottom.

On 29 April she contacted the French aviso Tahure guarding a freighter (Song Giang Maru, 1065 grt) off the Hon Doi Islands, and sank both.

After sinking a large cargo ship (Teisen Maru) in the Sulu Sea 9 May, Flasher set course for Fremantle, arriving 28 May for refit until 19 June.

Flasher made her third war patrol in the South China Sea, where on 28 June 1944 she contacted a heavily escorted convoy of 13 ships.

She made a cautious approach, undeterred by the escort, and shortly after midnight 29 June, broke into the convoy to sink a freighter (Nippo Maru 6,079 tons) and badly damage a large passenger cargo ship.

During her fourth war patrol, in the Philippines, Flasher headed a coordinated attack group which included two other submarines, Hawkbill and Becuna.

As she made her approach in a heavy downpour, a destroyer suddenly loomed up before her, and Flasher launched her first spread of torpedoes at this escort.

[n 1] Flasher contacted another well-guarded tanker convoy on the morning of 21 December 1944, and she began a long chase, getting into position to attack from the unguarded shoreward side.

Flasher′s conning tower was removed and placed on display as a memorial at the entrance to Nautilus Park, a Navy housing area in Groton, Connecticut.