First patrol

U-331's first patrol took her from Kiel in Germany on 2 July 1941, out into the mid-Atlantic, before arriving at Lorient in France on 19 August.^[4]

Second patrol

She sailed from Lorient on 24 September and headed into the Mediterranean Sea. There on 10 October she engaged three British tank landing craft off Sidi Barrani, Egypt. After missing with a torpedo, she engaged with her deck gun, slightly damaging HMS TLC-18 (A 18), before breaking off the attack after being hit by 40 mm shells, which wounded two men (one fatally) and damaged the conning tower. She arrived at Salamis, Greece, the next day, 11 October.^[5]

Third patrol-HMS Barham

Sailing from Salamis on 12 November 1941, U-331 returned to the Egyptian coast. On 17 November she landed seven men of the Lehrregiment Brandenburg^[6] east of Ras Gibeisa, on a mission to blow up a railway line near the coast, which failed.^[7] On 25 November 1941, north of Sidi Barrani, U-331 fired three torpedoes into the British Queen Elizabeth-class battleship HMS Barham. As the ship rolled over, her magazines exploded and she quickly sank^[8] with the loss of 861 men, while 395 were rescued.^[9] U-331 returned to Salamis on 3 December, where her commander, Freiherr Hans-Diedrich von Tiesenhausen, was subsequently promoted to Kapitänleutnant and awarded the Knight's Cross of the Iron Cross.^[10]

Fourth and fifth patrol

U-331 left Salamis on 14 January 1942 for another patrol off the Egyptian coast, this time with no success. She then sailed for La Spezia, Italy, arriving on 28 February.^[11]

Her next patrol was the reverse of the previous one, she left La Spezia on 4 April, patrolled the enemy coast, then returned to Salamis on 19 April.^[12]

Sixth to ninth patrol

Her next four patrols were similarly uneventful, operating from Messina, Sicily, and then La Spezia again, from May to September 1942, patrolling the North African coast without success.

Tenth patrol

U-331 departed La Spezia on her final voyage on 7 November 1942 to attack the massed ships of "Operation Torch".^[13] Two days later, on 9 November, U-331 sighted the American 9,135 GRT troopship USS Leedstown (AP-73) off Algiers. The Leedstown had landed troops on the night of 7/8 November, and the next day had been hit by an aerial torpedo from a Ju 88 torpedo bomber of III./KG 26 destroying her steering gear and flooding the after section. U-331 fired a spread of four torpedoes at the ship hitting her with two. Leedstown settled by the bow with a heavy starboard list, and was abandoned, finally sinking two hours later.^[14]

On 13 November U-331 was attacked by an escort ship and was slightly damaged when she dived too deep and hit the sea bed.^[13]

Wolfpacks

U-331 took part in one wolfpack, namely:

• Goeben (24 – 30 September 1941)