U-110 set out on her first patrol from Kiel on 9 March 1941. Her route to the Atlantic Ocean took her through the gap between the Faroe and Shetland Islands. Her first victim was Erdona which she damaged south of Iceland on 16 March. She also damaged Siremalm on the 23rd. This ship only escaped after she was hit by a torpedo which failed to detonate, (although it left a large dent) and the U-boat’s 105mm deck gun crew forgot to remove the tampion or plug in the muzzle before engaging their target. The resulting explosion on firing the first round wounded three men and compelled the boat to fire on the merchantman with the smaller 37 and 20 mm armament. Despite being hit, Siremalm successfully fled the scene, zig-zagging as she went.
U-110 arrived in Lorient on the French Atlantic coast on 29 March, having cut the patrol short due to damage from the exploding gun.
The boat departed Lorient on 15 April 1941. On the 27th, she sank Henri Mory about 330 nautical miles (610 km; 380 mi) west northwest of Blasket Islands, Ireland.
Her next quarry were the ships of convoy OB 318 east of Cape Farewell (Greenland). She successfully attacked and sank Esmond and Bengore Head, but the escort vessels responded. The British corvette, HMS Aubrietia, located the U-boat with ASDIC (sonar). Aubrietia and British destroyer Broadway then proceeded to drop depth charges, forcing U-110 to surface.
U-110 survived the attack, but was seriously damaged. HMS Bulldog and Broadway remained in contact after Aubrietia’s last attack. Broadway shaped course to ram, but fired two depth charges beneath the U-boat instead, in an endeavour to make the crew abandon ship before scuttling her. Lemp announced “Last stop, everybody out”, meaning “Abandon ship”. As the crew turned out onto the U-boat’s deck they came under fire from two attacking destroyers Bulldog and Broadway with casualties from gunfire and drowning. The British had believed that the German deck gun was to be used and ceased fire when they realised that the U-boat was being abandoned and the crew wanted to surrender.
Lemp realised that U-110 was not sinking and attempted to swim back to it to destroy the secret material, and was never seen again. A German eyewitness testified that he was shot in the water by a British sailor, but his fate is not confirmed. Including Lemp, 15 men were killed in the action, 32 were captured. Radio Officer, Georg Högel and the rest of the crew were held at Camp 23 (Monteith POW camp at Iroquois Falls, Northern Ontario, Canada), which is now the Monteith Correctional Complex.
Bulldog’s boarding party, led by sub-lieutenant David Balme, got onto U-110 and stripped it of everything portable, including her Kurzsignale code book and Enigma machine. William Stewart Pollock, a former radio operator in the Royal Navy and on loan to Bulldog, was on the second boat to board U-110. He retrieved the Enigma machine and books as they looked out of place in the radio room. U-110 was taken in tow back toward Britain, but sank en route to Scapa Flow.
The documents captured from U-110 helped Bletchley Park codebreakers solve Reservehandverfahren, a reserve German hand cipher.