Lemp commanded U-110 for her entire career. In an earlier command (U-30) he was responsible for the sinking of the passenger liner Athenia on the first day of the war. The circumstances were such that he was considered for court-martial. He continued, however, to be one of the most successful and rebellious commanders of his day.
U-110 conducted two patrols, sinking three ships totalling 10,149 tons and damaging two others totalling 8,675 tons. On 23 March, 1941, her 105 millimetre deck gun exploded during firing, wounding three men. On 9 May, 1941, she was captured and later sunk.
U-110 survived the attack, but was seriously damaged. After a second depth-charge attack, she surfaced, to the crew's relief, and 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 warships (HMS Bulldog and HMS 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 would surrender.
The escort commander, Captain Joe Baker-Cresswell in Bulldog, had initially made to ram, but recognising the opportunity for capture, pulled out and hove to, before strafing the submarine. Broadway also closed in, intending to prevent U-110 submerging and suffering incidental damage.
Lemp assumed that the boat, with vents open, would sink and ordered a radio operator, Heinz Wilde, to leave the codebooks and Enigma machine and get out - "The U-boat is sinking", he is reported to have said. Another radio operator recovered personal effects, ignoring the secret material.
Lemp realised that the U-110 was not sinking and attempted to swim back to it to destroy the secret material. He was never seen again. He may have been shot in the water by a British sailor (as testified by a German eyewitness), but his fate is unknown. Including Lemp, 15 men were killed in the action and 32 captured.
Bulldog's crew boarded U-110 and stripped it of everything portable on the spot, including her secret documents and Enigma cipher machine. U-110 was taken in tow back toward Britain, but sank en route to Scapa Flow.
The movie U-571 was partially inspired by the capture of U-110.