Brooklyn joined CruDiv 8 in 1938 and carried out the usual peacetime routine in the remaining years before the outbreak of war in Europe. She served on both the east and west coast in this period. In May 1941 she was in the Atlantic, and was later engaged on the transport of US troops for the occupation of Iceland. For the rest of 1941 she was engaged on Neutrality Patrols in the Atlantic. After the US entry into the war Brooklyn was assigned to convoy cover duties on the North Atlantic sea routes between Britain and the USA. In October 1942 she sailed for North Africa and Operation Torch, where, on 8 November, she provided fire support for the landings at Fedhala. In the course of this task she engaged the Vichy French cruiser Primaguet, together with Augusta, and badly damaged her. In the first half of 1943 she was employed in covering convoys between the USA and the North African battle zone, but in July she was assigned to support the landings in Sicily, Operation Husky, covering TF86 at Licata and the left flank of the US 7th Army. She then supported TF81 at the Salerno landings in September, and the same force at the Anzio landings in January 1944. Later that year Brooklyn provided a similar service for TF87 during the landings in the south of France in August. She returned to the USA at the end of November 1944, then went into refit at New York Navy Yard until May 1945. After refit Brooklyn was not assigned to the Pacific, but remained in the Atlantic. She was paid off on 3 January 1947, remaining in reserve until she was handed over to Chile on 9 January 1951 and renamed O'Higgins. On 12 August 1974 the ship was badly damaged by grounding, and was subsequently used only as a static harbour training ship, but in 1977/78 she was extensively refitted and returned to service. O'Higgins was finally decommissioned on 14 January 1992. On 3 November 1992 the ship foundered in tow to shipbreakers in the Far East.

Philadelphia was the second ship of the class to be completed, and also joined CruDiv 8, becoming flagship on 27 June 1938. Between the summer of 1939 and May 1941 she served in the Pacific, but was then transferred to the Atlantic, where she participated in Neutrality Patrols and in the occupation of Iceland. Thereafter, Philadelphia was employed in ocean convoy duties between the USA and Iceland, as well as to the UK, during the course of 1942. During this period she was also deployed to the Caribbean on patrol duties. By September 1942 she had been assigned to the Western Task Force for the North African landings, which took place on 7 November. The ship returned to the USA on 24 November, escorting convoys from the USA to the North African bridgehead until March 1943, and then joined TF85 in preparation for the invasion of Sicily, Operation Husky. The landings started on 10 July, and from that time until August the ship was tasked with gunfire support for the army ashore. After Husky, Philadelphia was assigned to the Salerno landings in September, bombarding Axis positions along the coast. During these tasks she was slightly damaged by near-misses from radio-guided bombs. By early November, however, she had been withdrawn to Oran, and sailed from there to the USA on 6 November. A short refit was carried out at New York Navy Yard in December/January, the ship then returning to the Mediterranean for gunfire support duties at Anzio from mid-February to the end of May 1944. For the landings in the south of France she was a part of TF85, then remained in Mediterranean waters until October. Philadelphia was refitted again at Philadelphia Navy Yard between November 1944 and May 1945, then served on duties between the USA and Europe until 3 February 1947, when paid off. On 9 January 1951 the ship was sold to Brazil and renamed Barroso, commissioning on 21 August 1951. Stricken from the operational list in 1973, the cruiser was finally broken up in Sao Paulo in 1974.

Savannah made several foreign cruises during her early years of service; to Cuba, Haiti and Great Britain. In June 1939 she sailed for the west coast, where she served until 19 May 1941, when ordered to the east coast again. On 17 June Savannah assumed the role of Flagship CruDiv 8, operating with the Neutrality Patrol in the Atlantic. She remained in the Atlantic after the US entry into the war, and in October 1942 was assigned to the Western Task Force for the invasion of North Africa. Following the landings, she returned to the USA and then operated in the South and central Atlantic on anti-blockade breaker patrols with the escort carrier Santee. By May 1943 she had returned to the Mediterranean for the invasion of Sicily, when she carried out shore bombardment tasks for which her floatplanes spotted, three of them being shot down. Then followed the Salerno landings, where, on 11 September, she was heavily hit by an FX1400 radio-controlled stand-off bomb launched from a German aircraft. This penetrated the roof of No. 5 turret and exploded in the magazine, blowing out the bottom of the ship in this area. The rapid ingress of water prevented an ammunition explosion, but flooded some 152ft of the ship, disabling the machinery at least in part for a period. There were many casualties, and the ship limped back to Malta for temporary repairs. It was not until 7 December that the ship was able to sail for the USA and full repairs. These lasted until September 1944, after which the ship was mainly employed on training duties on the east coast until the end of the war. Savannah was decommissioned on 3 February 1947, seeing no further active service before being stricken on 1 March 1959. For some reason the ship was not sold for scrapping until 25 January 1966.

Nashville served in the Atlantic Fleet from commissioning until June 1939, visiting the Caribbean and European waters during that period. The next two years were spent in the Pacific before the ship was ordered to the east coast once more in May 1941, arriving at Boston in mid-June for escort duties to Iceland. She then served on the Neutrality Patrol, based on Bermuda until March 1942, when ordered to the Pacific with the carrier Hornet, with whom she subsequently took part in the Doolittle raid on Tokyo on 17 April 1942. By May Nashville was operating in the North Pacific with TF8 in the Aleutians campaign, which lasted until November, the cruiser then being assigned to the south Pacific to join TF67 at Espiritu Santo as flagship. She participated in the final Guadalcanal actions at the beginning of 1943, shelling Munda and Vila. By May 1943 the ship was in action off New Georgia and in the Vella Gulf, once again shelling Munda, but on 12 May she suffered an explosion in No. 1 turret magazine, after which she sailed for the west coast for repair at Mare Island. Nashville arrived back at Pearl Harbor on 12 August 1943 and then accompanied carrier groups (TF15) for attacks on Marcus Island and Wake Island. In December she was part of TG74.2 for the landings at Cape Gloucester. In January 1944 Nashville was in action again during the landings at Saidor, New Guinea, bombarding Japanese troops as they attempted to evacuate to Madang. February saw the landings in the Admiralty Islands, when the cruiser was part of TF74, shelling the islands of Hauwei and Norilo into March. The following month she was a component of TF75 for the landings at Hollandia and Aitape, then at Biak in May. Her next major operation was the landings on Morotai on 15 September, as part of TG75.1. In December 1944 she was flagship of TG78.3 for the landings on Mindoro, but on the 13th she was hit by a kamikaze and badly damaged, suffering some 310 dead and wounded. She was withdrawn to Puget Sound for repairs which took from January to April 1945, after which she sailed for the Philippines as Flagship TF74. In the final months of the war she operated in the Borneo campaign, for the landings in Brunei Bay. After the war she went to Chinese waters, then assisted in bringing home US troops until paid off on 24 June 1946. On 9 January 1951 she was sold to Chile and renamed Capitan Prat. In 1982 she was renamed Chacabuco, and was decommissioned in 1984 to be scrapped in Taiwan the following year.

Phoenix joined CruDiv 9 with the Battle Force and served in the Pacific, being present in Pearl Harbor during the Japanese attack but escaping damage. She was immediately sailed to find the Japanese fleet, but this was unsuccessful. In the following months she was employed on ocean convoy escort duties to the USA and to Australia, spending a period in the Indian Ocean early in 1942 and then working with ANZAC forces, TF44, in the south-west Pacific into 1943. In March 1943 TF44 became TF74 on the formation of the 7th Fleet, but in April Phoenix was relieved and, in July, she started a refit at Philadelphia. Then, after a trip to the North African coast, she was reassigned to the 7th Fleet in the Pacific. She returned in time to participate in the landings at Cape Gloucester as part of TG74.2 at the end of December, and then provided support for landings in New Guinea in January. She subsequently operated in the Admiralty Islands, at Hollandia and Biak, giving gunfire support to landing operations. In September she was at the landings at Morotai, and in October the ship was part of the forces committed to the invasion of Leyte as a unit of TG73.3. In January 1945 Phoenix covered the invasion of Luzon, then Bataan and Corregidor in February and Mindanao in March. The remainder of the war was spent in the Philippines and Borneo area before the ship returned to Pearl Harbor on 15 August 1945. After her return home she was finally paid off on 3 July 1946. On 12 April 1951 she was sold to Argentina, refitted and recommissioned as 17 de Octubre on 17 October 1951. In 1956 she was renamed General Belgrano. On 3 May 1982 she was torpedoed and sunk by the British submarine Conqueror off the Falklands Islands.

Boise was another ship to join CruDiv 9 with the Battle Force in the Pacific. She was in the Pacific at the start of the war in the Far East, but missed the critical battles in the Dutch East Indies as a result of grounding on 21 January 1942, which necessitated repair at Mare Island. After her return to action she was used mainly on ocean convoy duties in the Pacific until committed to the Guadalcanal campaign in August 1942. However, at the Battle of Cape Esperance on 11 October she was severely damaged in action with Japanese cruisers and destroyers, although her gunfire assisted in the sinking of the destroyer Fubuki. The cruiser Aoba and destroyer Hatsuyuki were also damaged. A shell hit her below the waterline belt and caused fires in the forward turret shell handling room, but without an explosion. The cruiser was under repair at Philadelphia Navy Yard between 19 November 1942 and 20 March 1943. After repair she was transferred to the Mediterranean for the landings in Sicily and at Salerno, but left the Mediterranean again on 8 June for the Pacific. In December Boise arrived off New Guinea and then served in the Philippines and Borneo area until July, when she arrived back in San Pedro. Boise was decommissioned on 1 July 1946 and sold to Argentina on 12 January 1951, being officially transferred on 12 April 1951. She recommissioned as Nueve de Julio on 11 March 1952. The cruiser was stricken in 1979 and scrapped at Brownsville, Texas, in 1983.

Honolulu served as Flagship, Cruisers Battle Force, and Flagship CruDiv 9 in the Pacific from completion, and was at Pearl Harbor at the time of the Japanese attack, when she suffered minor damage. Thereafter she was employed on ocean convoy escort duties between the USA, Australia and the western Pacific until May 1942, when she was assigned to the Aleutian campaign. In November, after a refit at Mare Island, Honolulu joined the Guadalcanal campaign and was present at the Battle of Tassafaronga, escaping damage in that disastrous action. She remained in the Guadalcanal theatre until its conclusion early in 1943. By May she was with TF68 off New Georgia, and in July was in the Kula Gulf as flagship of TF36.1. During the Battle of Kula Gulf on 5/6 July she was one of the cruisers responsible for the sinking of the destroyer Niisuki, but her sister Helena was also lost in this exchange of fire. A few days later, in the Battle of Kolombangara, her gunfire and that of St Louis sank the cruiser Jintsu, but then she was herself hit by one dud torpedo from Japanese destroyers and one which exploded, wrecking the entire bow forward of the capstans. The ship had to return to the USA for repairs, and did not return to service until November 1943. By February 1944 she was with TF38 off New Ireland, and operated in the Solomon Islands until June. The ship then participated in the assault on Saipan as part of TG52.10. As the year progressed she saw action off Guam, Palau and Marcus Island before the invasion of the Philippines in October. At the landings in Leyte Gulf, Honolulu was hit by an air launched torpedo on 20 October and badly damaged. This effectively ended her war service, as repairs lasted until October 1945. She was paid off on 3 February 1947, and finally sold for breaking up on 17 November 1959.

St Louis served on the Neutrality Patrol in the Caribbean until September 1940, but in December was transferred to Pearl Harbor, where she was still stationed at the time of the Japanese attack. In January she participated in the attack on the Marshall Islands with TF17, after which she carried out ocean convoy escort duties between the US west coast and the south Pacific until May, then being reassigned to the Aleutians campaign. She was active in the North Pacific until recalled for refit at Mare Island in October 1942. Early in December she returned to the south Pacific to operate in the Solomon Islands and the Guadalcanal campaign. In June 1943 she took part in the landings on New Georgia, covering operations in the Kula Gulf with TF36.1. On the night of 5/6 July she took part in the Battle of Kula Gulf, when her sister Helena was sunk, and on 12/13 July during the Battle of Kolombangara she was herself hit by torpedoes from Japanese destroyers, which took off her bows below the second deck. Following provisional repairs at Tulagi and Espiritu Santo, the cruiser departed for Mare Island and full repairs, which took until November. She returned to action at Bougainville, then operated in the Shortland Islands, Solomons, Bismarcks and New Ireland as part of TF39. On 14 February 1944, while in the Solomons, she was hit by bombs and damaged. By May the ship was with TF52, and participated in the landings at Saipan in June, and in the Battle of the Philippine Sea in mid-June. Between July and October she was again in refit at a US yard, but was back off Leyte Gulf by mid-November, only to be hit by a kamikaze on 27 November. This necessitated repairs in the USA, which lasted until March 1945. St Louis arrived back in the south Pacific at Ulithi in March, to participate in the carrier raids on Japan and in the Okinawa campaign, then operated in the South China Sea with TF95, joining TF73 off Shanghai in August. After repatriation duties in 1945/46, the ship was decommissioned on 20 June 1946, to be sold to Brazil on 22 January 1951 and transferred on 29 January as Tamandare. She was stricken in 1975 and sold for scrapping. Towed from Rio De Janeiro on 5 August 1980, she foundered in tow off South Africa on 24 August 1980, while en route to shipbreakers in Taiwan.

Helena had a brief but eventful career, joining the Pacific Fleet on completion. At Pearl Harbor on 7 December 1941 she was hit by a torpedo and had to return to the USA for repairs. She subsequently saw service in the Guadalcanal campaign, assisting in the rescue of survivors from Wasp. In October, at the Battle of Cape Esperance, she took part in the sinking of the cruiser Furataka and the destroyer Fubuki. During the Battle of Santa Cruz she was a component of TF64. She survived the action off Savo Island on 12 November, when Atlanta and four destroyers were lost, then in the new year took part in bombardments of New Georgia. For the final battles of Guadalcanal she was with TF67, bombarding Japanese positions. On 11 February 1943 she was attacked by the submarine I 18, but this boat was herself destroyed off Espiritu Santo by the cruiser's escort, Fletcher and O'Bannon, with the assistance of the ship's aircraft. After a refit at Sydney, Helena returned to TF68 off New Georgia in March, but at the Battle of Kula Gulf on 5/6 July she was torpedoed and sunk by Japanese destroyers, with many casualties.