At the outbreak of the Second World War, Müllenheim-Rechberg worked on different battleships. In 1941, he experienced the sinking of the battleship Bismarck as fourth artillery officer with the rank of captain lieutenant, thereby becoming the highest-ranking survivor of the ship. He was rescued by the British cruiser "Dorsetshire" and remained in a prisoner of war camp in Ontario, Canada, until the end of 1946. On 1 February 1943, he was promoted to corvette captain.