First, second and third patrols

On 24 June 1941, nineteen days into her first patrol while en route from Kiel to St. Nazaire in France, U-203 attacked and sank the GRT 4,956 GRT British merchant ship Kinross, which was part of Convoy OB 336. Later that day, she attacked the 4,402 GRT Norwegian ship Soløy, which was with Convoy HX 133.

Arriving at St. Nazaire on 29 June, the crew of U-203 stayed ashore for another eleven days before the boat made her second patrol. Seventeen days into it, she attacked Convoy OG 69 which was sailing to Gibraltar. On the 27th, U-203 sank the 2,475 GRT British merchant ship Hawkinge. The British Lapland and the Swedish Norita, also OG 69 merchant ships, were sunk the following day, adding a further 1,330 GRT and 1,516 GRT respectively to the U-boat's record.

Six days into her third patrol she joined a wolfpack attack on Convoy HG 73 On 26 September she sank the British ships Avoceta and Lapwing and the Norwegian Varanberg, destroying another 7,362 GRT of shipping. Avoceta's sinking killed 123 people. The Convoy Commodore was aboard, but was one of the few dozen survivors.

Fourth, fifth and sixth patrols

Sixteen days into her fourth patrol, on 3 November, U-203 attacked and sank Empire Gemsbuck (5,626 GRT) and Everoja (4,830 GRT), British merchant ships of Convoy SC 52. Everoja was Latvian owned and carried an Irish cargo. However, she transferred to the British registry and sailed under the Red Ensign.^[3] Twenty-one days into her fifth patrol on 15 January 1942, U-203 sank a small (632 GRT) Portuguese ship, Catalina. Two days later, the Norwegian ship Octavian would also be sunk. It was never confirmed that the Octavian was sunk by the U-203. It was surmised, the Octavain was actually never in the area where the U-203 said it was torpedoed. The Octavian was positively identified and found on 4 July 2018 off the coast of Maryland. Four torpedoes were fired at the Canadian ship North Gaspe, which survived the attack. One torpedo detonated close to the ship, but none actually hit.

U-203's sixth patrol resulted in a total loss of 14,232 GRT to the Allies and two additional ships damaged, including the British merchant ships San Delfino and Empire Thrush. Damaged were the American tanker Henry F. Sinclair, Jr. and the Panamanian flagged tanker Stanvac Melbourne.

Seventh, eighth and ninth patrols

A seventh patrol beginning 4 June 1942, resulted in 32,985 GRT of shipping destroyed. The Brazilian Pedrinhas and the British Putney Hill were sunk on 26 June. Two days later the American Liberty Ship Sam Houston was torpedoed and then finished off with 43 rounds from the deck gun. The British Cape Verde was sunk on 9 July and the Panamanian tanker Stanvac Palembang on 11 July bringing the tally to five victories on this successful patrol.

The next 23-day sortie was unfruitful; however, the U-boat's commander, Rolf Mützelburg, died during this patrol on 11 September. Taking the opportunity to go swimming in the Atlantic southwest of the Azores, he dived from the conning tower and struck the deck with his head and shoulder when the U-boat lurched suddenly in the swell. The doctor from U-462, a 'Milk Cow' supply submarine, arrived the next day, but too late. Rolf Mützelburg was buried at sea on 12 September 1942 in position 36°14′N 31°21′W. He was replaced by Kptlt. Hermann Kottman, who served as captain for the remainder of U-203's career.

Her ninth patrol, beginning 15 October 1942 and terminating 6 November, yielded two further ships sunk. The British merchantmen Hopecastle and Corinaldo (5,178 and 7,131 GRT, respectively) were sunk on 29 October and 30 October while traveling with Convoy SL-125.

Wolfpacks

U-203 took part in eleven wolfpacks, namely:

• Schlagetot (20 October – 1 November 1941)
• Raubritter (1 – 5 November 1941)
• Seydlitz (27 December 1941 – 7 January 1942)
• Zieten (7 – 22 January 1942)
• Iltis (6 – 10 September 1942)
• Streitaxt (20 – 30 October 1942)
• Raufbold (11 – 22 December 1942)
• Spitz (22 – 31 December 1942)
• Lerche (10 – 16 April 1943)
• Meise (16 – 22 April 1943)
• Specht (23 – 25 April 1943)