St. Louis was the only true sister of Helena. They were the last two Brooklyn-class light cruisers, built to a modified design that is often called the St. Louis subclass.
Both carried fifteen 6-inch/47 guns in five triple turrets and were limited to about 10,000 tons standard under the London Naval Treaty. They differed from the earlier Brooklyns in two ways. The machinery was arranged in a unit system, boiler rooms and engine rooms alternating, so one hit was less likely to knock out all power. The secondary battery was eight 5-inch/38 dual-purpose guns in four enclosed twin mounts, instead of the older open 5-inch/25 mounts.
Newport News Shipbuilding laid her down on 10 December 1936, launched her on 15 April 1938, and commissioned her on 19 May 1939. After shakedown she spent about a year on Atlantic Neutrality Patrol, including a September 1940 inspection cruise from Newfoundland to British Guiana tied to the Destroyers for Bases deal. She shifted to the Pacific in late 1940.
She was at Pearl Harbor when the Japanese attacked on 7 December 1941. She was one of the few major ships to get underway during the raid and clear the harbor under fire.
In 1942 she worked the Aleutians, including the bombardment of Kiska, then moved to the South Pacific for the end of the Guadalcanal campaign and the drive into the central Solomons. She and Helena often operated together. On the night of 5–6 July 1943, in the Battle of Kula Gulf, their force intercepted a Japanese destroyer reinforcement run. Helena was hit by three torpedoes and sank. St. Louis came through. Six nights later, in the Battle of Kolombangara, a torpedo hit her well forward and twisted her bow. She made it back to California for repairs.
She returned in late 1943 and stayed in the Pacific through the rest of the war: Bougainville, the Marianas and the Battle of the Philippine Sea, Leyte Gulf, carrier raids on Japan, and the Okinawa campaign. She earned eleven battle stars.
She was decommissioned on 20 June 1946 and laid up at Philadelphia. In January 1951 she was sold to Brazil, struck from the U.S. Navy list, and recommissioned as Almirante Tamandaré. She served the Brazilian Navy until 1976. In 1980, while under tow to be scrapped, she sank.
Helena, the second of the pair, never got that long career. She was lost at Kula Gulf on 6 July 1943, with heavy casualties, and is remembered as one of the hardest-fighting light cruisers of the Pacific war.