Eldorado (AGC-11) was launched on the 26th of October, 1943 as the "SS Monsoon" by North Carolina Shipbuilding company, Wilmington, North Carolina, under a Maritime Commission contract; sponsored by Mrs. P.A. Peeples. The ship was transferred to the Navy on the 1st of February 1944 and converted by Bethlehem Steel Corporation of Brooklyn New York. She was commissioned on August 25th, 1944 with Captain J.R. Wallace in command.

The Eldorado sailed from Norfolk on September 15, 1944 and arrived at San Diego on September 29th to embark Rear Admiral L.F. Reifsnider who broke his flag as Commander, Amphibious Group 4. In November Eldorado sailed to Pearl Harbor and there became flagship for Vice Admiral R.K. Turner, Commander, Amphibious Forces, Pacific. 

After rehersal landings in Hawaii, the command ship sailed January 27th 1945 for the Marianas and further preparations for the assault on Iwo Jima. She also carried General H.M. Smith, USMC, and his staff, and Secretary of the Navy J.V. Forrestal and his party when she sailed from Saipan February 16th for Iwo Jima. From February 19th to March 9th, Eldorado lay off Iwo Jima, her distinguished passengers directing operations ashore and afloat. She served as headquarters for war correspondents, and broadcasted directly from the beachhead to the people at home through her facilities. Through the critical period of this bloody and arduous operation, she carried out her duties as flag ship and operations center with effective thoroughness.

Arriving at Guam on March 12th, 1945, Eldorado embarked Lieutenant General S.B. Buckner, USA, Commanding General of the 10th Army. After rehearsals at Leyte in the Philippines, hove to off the Hagushi Beaches, Okinawa for the initial invasion landings on 1 April. Here she carried out with equal distinction the same type of duties she had performed at Iwo Jima. Since both the Commander, Air Support Control Unit, and the Force Fighter Director Officer was embarked, Eldorado's combat information center was the central unit in the air defense against the day and night air raids. General Buckner and his staff debarked April 18th to establish headquarters on the island itself, and until the ship's departure May 18th, she was visited by several distinguished guests, including Admirals Chester W. Nimitz, William F. Halsey, Jr, and R.A. Spruance, and the noted war correspondent Ernie Pyle.

At the end of the war Eldorado was at Manila preparing for the proposed invasion of the Japanese home islands. She returned to Pearl Harbor in October where Admiral Turner and his staff debarked.

Alternately at Pearl Harbor and at west coast ports, Eldorado continued to serve as flagship for succeeding amphibious commanders in the Pacific. There were two exceptions: From April to September 1947, and again from January to July 1949, she flew the flag of Commander, Naval Forces, Western Pacific, and cruised to Chinese waters. During the second tour, she departed Shanghai only a short time before that city fell to the Communists.

With the outbreak of the Korean war, Eldorado was ordered to the Far East. As flagship for Rear Admiral L.A. Thackrey, Commander, Amphibious Group 3, she acted as standby for Mount McKinley (AGC-7) during the invasion of Inchon, Korea, and coordinated and controlled the logistics operations. In October 1950, she moved to Iwon to support the continued northwest advance of United Nations troops. Returning to Japan in November, she was ordered again to Inchon to direct the evacuation. She was at Inchon again in the spring and summer of 1951, and in June hoisted the flag of Vice Admiral I.N. Kiland, Commander, Amphibious Forces, Pacific Fleet. She was visited by Generals Ridgeway and Van Fleet, and the commanding generals of the British troops and Turkish Brigades during her stay at Inchon, and sent the first pictures of the Korean truce talks to the outside world.

Returning to the States, Eldorado became flagship for Rear Admiral W.E. Moore, Commander, Amphibious Group 1, in October 1952, and sailed for the Far East where Admiral Moore assumed command of TF 90's amphibious forces. During this tour she assisted the Japanese Government during the floods at Fukuoka, and directed Operation "Big Switch", the transportation of Chinese and Korean prisoners of war from the camps at Cheju Do and Koje Do to the port of Inchon for repatriation.

Eldorado returned to the west coast in the fall of 1953, and continued to serve as flagship for Amphibious Group 1, now commanded by Rear Admiral L.S. Sabin Jr, until June 1954, then embarked Vice Admiral T.G.W. Settle, Commander, Amphibious Forces, Pacific, until August. On 15 February 1955 she sailed for Keeling, Formosa, where she operated as flagship for Vice Admiral A.M. Pride, Commander, 7th Fleet, until August 17th. She returned to San Diego for amphibious exercises, and on December 13th Rear Admiral G.C. Towner broke his flag on board as Commander, Amphibious Group 3 and Eastern Pacific.

In the summer of 1956, Eldorado sailed to arctic waters with Army officers embarked, to resupply bases at Wainwright and Point Barrow, Alaska. From December 1956 to January 1957, she was flagship for Vice Admiral C.F. Espe, Commander, Amphibious Forces, Pacific, and during the next month, for Vice Admiral R.L. Denison, Commander, First Fleet. From June 1957 through 1960, she served as flagship for four successive commanders of Amphibious Group 1, Rear Admirals F.C. Stelter Jr, C.K. Duncan, C.O. Triebel and C.C. Kirkpatrick. The only interruption to this service was in October and November 1958, when she served Vice Admiral J. Sylvester, Commander, Amphibious Forces, Pacific, as flagship. In addition to operations along the west coast from her home port at San Diego, Eldorado took part in arctic supply operations once more in the summer of 1957, and cruised to the Far East from January to May 1958, and again from December 1959 into 1960. During the second of these, as flagship, Eldorado participated in exercise "Blue Star", a joint Navy-Marine and Nationalist Chinese navy-marine amphibious operation off southern Taiwan. In April and May, Eldorado visited ports in Indonesia and Australia, participating in Australia's 18th annual celebration of the Battle of the Coral Sea before returning to San Diego, on May 31st where she remained, participating in local operations through 1962.