USS San Juan, a 6000-ton Atlanta class light cruiser built at Quincy, Massachusetts, was commissioned at the end of February 1942. In June of that year, following shakedown in the western Atlantic, she went to the Pacific to join the war against Japan. She arrived in the south Pacific in July, in time to particiate in the initial phases of the prolonged and bitter Guadalcanal Campaign. The cruiser provided gunfire support when U.S. Marines landed on Guadalcanal and Tulagi on 7 August, and served thereafter mainly as an escort for U.S. aircraft carriers. In October 1942, she raided Japanese shipping in the Gilbert Islands and served in the screen during the Battle of the Santa Cruz Islands. San Juan was hit by a Japanese bomb during the latter action, requiring her to steam to Australia for repairs.

For the rest of 1942 and most of 1943, as the fight for Guadalcanal reached a climax and the war moved up through the Central Solomons, San Juan operated with the carriers in the south Pacific. She accompanied USS Saratoga (CV-3) during November 1943 strikes on enemy targets on Bougainville and Rabaul and in covering the invasion of the Gilbert Islands. In early December, she took part in a carrier raid into the Marshall Islands.

After a west coast overhaul, San Juan supported the Marshall Islands invasion in January-February 1944, the extensive attacks on Japanese bases in the central Pacific from then into May, and landings at Hollandia, New Guinea, in April. She served with the carriers through the Marianas operation and the resulting Battle of the Philippine Sea in June and July 1944, and during attacks on the Bonin and Volcano Islands.

San Juan received shipyard work during August-October 1944, returning to the war zone in late November. From then until the War's end in August 1945, she kept busy in her usual role as aircraft carrier escort, taking part in strikes throughout the western Pacific in December 1944 - January 1945, the invasions of Luzon, Iwo Jima and Okinawa in January-May 1945, and raids against the Japanese home islands in February and July. After Japan's mid-August capitulation, San Juan supported the recovery of Allied prisoners of war and other occupation activities. Later in 1945 and into 1946, she brought U.S. servicemen home from the south Pacific as part of Operation "Magic Carpet". Inactivated during 1946, she was formally decommissioned in November of that year. San Juan was redesignated CLAA-54 in 1949, during her long stay in "mothballs". She was stricken from the Navy list in March 1959 and sold for scrapping in October 1961.