First patrol

U-256's first patrol began on 28 July 1942 during her transfer from Kiel, Germany to the 9. Unterseebootsflottille at Brest in occupied France.

During the patrol, U-256 unsuccessfully pursued Convoy SC 94, and early on 25 August was detected by the radar of the Norwegian Flower-class corvette HNoMS Eglantine astern of convoy ON 122. The U-boat crash-dived when illuminated by star shells, the corvette attacked with depth charges. The boat eventually made her escape when the corvette was obliged to return to convoy protection duties, but the submarine was damaged enough to have to abort the patrol.

On the return journey, on the morning of 2 September, the U-boat was attacked by a British Whitley bomber of No. 77 Squadron RAF in the Bay of Biscay. The aircraft strafed and dropped several bombs, but the U-boat's flak hit the aircraft and it crashed into the sea. U-256 limped into Lorient the next day, due to the extensive damage from the two attacks, she was withdrawn from service in November 1942.^[4]

Second patrol

During an overhaul which included extensive repairs, U-256 was converted to a Flakboot (Flak boat) in May 1943. One of just four U-boats so modified, she was given an increased complement of anti-aircraft guns, to give her and other German submarines a better chance of fighting off enemy aircraft. U-256 was re-commissioned as U-flak 2 on 16 August 1943.

The boat's second wartime patrol started on 4 October under the command of Oblt.z.S. Wilhelm Brauel; her mission was to rendezvous with and protect U-488, a Milchkuh ('Milk cow' or re-supply U-boat). Such submarines could re-supply multiple U-boats at sea, and were consequently the prime target of Allied aircraft trying to disrupt U-boat activities.

On 8 October, the outbound boat was attacked by a Leigh light-equipped British Wellington bomber of No. 612 Squadron RAF in the Bay of Biscay. U-256 was not damaged by the six depth charges that straddled her, and escaped by crash-diving. U-256's return fire had hit the starboard elevator and rear turret of the aircraft, but it returned safely to base.^[5]

U-flak 2 was caught on the surface by the American destroyer USS Borie on 31 October. The U-boat escaped, but with minor depth charge damage.

On 16 November the inbound boat encountered a Halifax Mk.II aircraft of No. 502 Squadron RAF in the Bay of Biscay. The aircraft was damaged by flak and turned away. When the Halifax returned to the area, the U-boat had already escaped by crash-diving.^[5]

U-flak 2 returned to her base at Brest on 17 November. The Flak conversion was not considered a success, U-flak 2 was converted back to her original configuration in the winter of 1943–44 and renamed U-256.

Third patrol

U-256's third patrol started on 25 January 1944. She steamed west and southwest of Ireland and on 20 February damaged the anti-submarine sloop, HMS Woodpecker which later sank while under tow.^[6] On 19 March, inbound in the Bay of Biscay, the U-boat was strafed by a Leigh light-equipped British Liberator of 224 Squadron. The Germans observed hits from their 20 mm and 37 mm AA guns, before the aircraft dropped six depth charges, then crashed 500 meters away. The boat was not damaged in the attack, and returned to Brest on the 22nd.^[7]

Fourth patrol

U-256 sailed from Brest on 6 June 1944 "(D-Day)", but was seriously damaged when attacked by another Liberator, also of 224 Squadron the next day, and returned to Brest on the eighth.^[8]

Fifth patrol

On 4 September 1944, under the command of Korvettenkapitän Heinrich Lehmann-Willenbrock, U-256 became the last U-boat to leave Brest before the port was captured by the Allies. The transit from Brest to Bergen in Norway, would be U-256's last patrol. She reached her destination on 17 October, and was decommissioned there on 23 October.^[9]

Wolfpacks

U-256 took part in five wolfpacks, namely:

• Steinbrinck (7 – 11 August 1942)
• Lohs (11 – 25 August 1942)
• Igel 2 (3 – 17 February 1944)
• Hai 1 (17 – 22 February 1944)
• Preussen (22 February - 13 March 1944)