[6] Harder performed shakedown off the United States East Coast, then headed for Pearl Harbor, Hawaii, via the Panama Canal.

While crossing the Caribbean Sea on the surface in a designated safety lane on 2 May 1943, she sighted an approaching U.S. Navy PBY Catalina flying boat at a range of 5,000 yards (4,600 m).

She made a radar approach on the surface and fired four torpedoes at the two-ship convoy, hitting the seaplane transport Sagara Maru (7,189 BRT) (which was beached to prevent sinking, but later destroyed).

Harder began her second war patrol 24 August 1943 from Pearl Harbor, and after touching at Midway Island, she again headed for the Japanese coast.

While patrolling off Honshū on 9 September, she attacked and sank Koyo Maru and later that night ran by an escort ship at a range of 1,200 yards (1,100 m) without being detected.

Submerging again until sunset, the submarine then surfaced under the cover of darkness and sank the damaged ship with gunfire, then turned toward Saipan in search of new targets.

Returning to action in the Pacific, Harder reached Pearl Harbor on 27 February 1944 and departed on her fourth war patrol on 16 March in company with Seahorse.

On 13 April, an enemy plane sighted Harder north of the western Carolines and reported her position to the patrolling Japanese destroyer Ikazuchi.

Then, adding to the enemy's losses, she returned to Woleai where she surfaced on the morning of 20 April to deliver a shore bombardment under cover of a rain squall.

Assigned the area around the Japanese fleet anchorage at Tawi-Tawi, Harder departed Fremantle on 26 May 1944 with Redfin and headed for the Celebes Sea.

On 6 June Harder entered the heavily patrolled Sibutu Passage between Tawi-Tawi and North Borneo and encountered a convoy of three tankers and two destroyers.

Following the inevitable depth charge attack, Harder transited the Sibutu Passage after dark and steamed to the northeast coast of Borneo.

Soon afterward, she underwent the inevitable depth charge attack by enemy planes, then she set course for a point south of Tawi-Tawi to reconnoiter.

On the afternoon of 10 June Harder sighted a large Japanese task force, including three battleships and four cruisers with screening destroyers.

As the range closed to 1,500 yards (1,400 m), she fired three torpedoes on a "down the throat" shot, then went deep to escape the onrushing destroyer and certain depth charge attack.

The deafening explosions shook the submarine far worse than the depth charges and aerial bombs that the infuriated enemy dropped during the next two hours.

On 21 August Harder and Haddo joined Ray, Guitarro, and Raton in a coordinated attack against a convoy off Palawan Bay, Mindoro.

Enemy trawlers towed the stricken destroyer to Dasol Bay, and Haddo, her torpedoes expended, informed Harder and Hake the following night of the attack and left the wolfpack for replenishment at Biak.

Before dawn on 24 August two ships were spotted which they initially identified as a Japanese minesweeper and the three-stack Siamese destroyer Phra Ruang.

PB-102 was built in the United States and commissioned as USS Stewart, a Clemson-class destroyer, but she was damaged by Japanese fire during the Battle of Badung Strait.

While under repair in drydock at Surabaya in February 1942, the Japanese captured the airfield at Bali, thus threatening the naval base and so the ship was scuttled at the docks early in World War II.

On 22 May 2024, Tiburon Subsea CEO Tim Taylor and the Lost 52 Project announced that they had discovered the wreck of Harder in the South China Sea near the Philippines' northern island of Luzon.

[13] A large hole on the port side just aft of the conning tower indicates Harder likely received a direct hit by a depth charge, similar to the fate suffered by Lagarto.