Like her sister ships, "Köln" served as a training ship for naval cadets in the 1930s, and joined the non-intervention patrols during the Spanish Civil War during the latter part of the decade. After the outbreak of World War II in September 1939, she conducted several operations in the North Sea, but did not encounter any British warships. She participated in the attack on Bergen during Operation Weserübung in April 1940, and she was the only member of her class to survive the operation. In 1942, she was modified to carry a Flettner Fl 282 helicopter experimentally. Later in 1942, she returned to Norway, but did not see significant action. She remained there until early 1945, when she returned to Germany; in March, she was sunk by American bombers in Wilhelmshaven. She remained on an even keel, with her gun turrets above water; this allowed her to provide gunfire support to defenders of the city until the end of the war in May 1945.