With his parents' approval he left Latvia to join the German Reichsmarine (renamed Kriegsmarine in 1935) on 1 April 1933 as an officer candidate.

He initially served with the surface fleet, going on a nine-month training tour around the world in the cruiser Karlsruhe from 24 September 1933 to 27 June 1934.

He sailed on a patrol in Spanish waters during the civil war in that country on the U-boat tender Erwin Wassner (13 April 1939 – 18 May 1939).

[5] Other sinkings included the surfaced French submarine Doris during the night of 8–9 May 1940 and seven merchant ships with a total of 16,669 gross register tons (GRT).

[7] On 27 June 1940 Lüth took command of U-138, a Type IID submarine, with which he sank four ships on his first patrol, totalling 34,644 GRT.

[14] U-43 was due to depart Lorient on a war patrol to an area off Freetown, west Africa, but early on 4 February 1941, she sank while tied to Ysere, an old sailing ship (which had carried the Statue of Liberty to New York in 1885) which was used as a floating pier.

To make matters worse and contrary to a Befehlshaber der U-Boote (BdU—U-boat command headquarters) directive, a hatch had been left open, allowing water to pour into the aft torpedo room.

After repairing his vessel, Lüth led it to Lourenco Marques and for the next fortnight U-181 undertook a series of surface attacks which resulted in eight ships being sunk, most primarily with U-181's deck gun.

[20] On 31 January 1943, Lüth and other Kriegsmarine officers traveled to the Wolf's Lair, Hitler's headquarters at Rastenburg, present-day Kętrzyn in Poland, for the Oak Leaves presentation.

During this meeting, Hitler appointed Dönitz as Oberbefehlshaber der Marine (Commander-in-Chief) of the Kriegsmarine following Raeder's resignation on 30 January 1943.

(The longest combat patrol of World War II was 225 days in length, which was achieved by Eitel-Friedrich Kentrat as commander of the U-196.)

[24] On 15 July 1943, Lüth sunk the British collier Empire Lake and noted in his logbook: "Five men have been left floating on a piece of wreckage.

The chief engineer Kapitänleutnant Carl-August Landfermann[26] and 2nd Watch Officer Johannes Limbach[27] both received the Knight's Cross for their achievements.

Returning drunk in the night of 13/14 May 1945,[28] Lüth failed to respond to the sentry's challenge and was shot in the head by 18-year-old seaman Mathias Gottlob, a German guard.

[1] Dönitz contacted the British commander of the city of Flensburg, asking him for permission to conduct a formal state funeral, which was approved by royal assent.

During the court martial, Gottlob stated that, in accordance with his orders, he had asked for the password three times without receiving a response from the person, whom he could not visually identify in the darkness.

[1] Lüth was the subject of a hagiographic account by the German author Franz Kurowski, published in 1988 under the pen name Karl Alman, commemorating "the most successful U-boat commandant of the Second World War" (according to the subtitle).

[30] In his 1995 book Count Not the Dead: The Popular Image of the German Submarine, Hadley panned Kurowski's works as "hackwork" and "pulp-trade yarn" focused on hero making.