The keel of LST 603 was laid in December 1943 by the Chicago Bridge and Iron Works, Seneca, Illinois, and launched on 14 March 1944; her sponsor was Mrs. Etla Nelson Hobart. She was commissioned on 5 April 1944 at New Orleans, Louisiana. Shortly afterwards she conducted a shakedown training at Panama City, Florida, and then proceeded to Norfolk, Virginia. The vessel departed Norfolk on 13 May 1944 in a convoy of 126 ships bound for Oran, Algeria. In the European Theater of Operations the ship supported the United States 7th Army and Free French 1st Army in the initial landings in Southern France. During December 1944 she supported the 1040th Construction Battalion Unit while it was building a PT base at Leghorn, Italy. Through March and April 1945 she participated in lifting the Canadian 1st Army from Leghorn to Marseilles. Following this she assisted in closing out U.S. Naval bases in the Mediterranean, including Bizerte, Tunisia, and La Moddalena, Sardinia. Early in June 1945, LST 603 sailed from Oran to Taranto, Italy then Marshall Tito was threatening the newly won European peace. On 26 June 1945 the ship proceeded from Oran via Casablanca to the United States. She arrived at the Brooklyn Navy Yard, New York City on 14 July 1945 and spent the remainder of the year undergoing overhaul and refresher training.

In January 1946 LST 603 joined the Amphibious Force, U.S. Atlantic Fleet and served with it until decommissioned on 12 May 1955. During the intervening period she participated in many exercises providing training for other amphibious ships, the Fleet Marine Force, Midshipmen from the U.S. Naval Academy and NROTC colleges, and naval reservists. These duties extended her range of travel to include Southern California via the Panama Canal; the Caribbean Islands, including Vieques and Puerto Rico; the Gulf of Mexico; the Atlantic Coast of the United States from Florida to Massachusetts; Hamilton Inlet, Labrador; and Thule, Greenland. Altogether the ship completed a total of 11 years and one month in continuous commissioned service.

The name COCONINO COUNTY, commemorating a northwestern Arizona county on the southern rim of the Grand Canyon, was assigned on 12 May 1955, the day the ship was decommissioned, and became effective on 1 July 1955.

After a yard overhaul/reactivation period of approximately five months at Camden New Jersey USS COCONINO COUNTY (LST 603) was recommissioned on 8 June 1966 at the Philadelphia Naval Shipyard, Philadelphia, Pennsylvania. After a brief training period in Little Creek Virginia, the ship sailed for the Pacific Ocean where she joined the U.S. Seventh Fleet as a unit of Landing Ship Squadron Three of the Amphibious Force, U.S. Pacific Fleet. The ship was homeported at Guam in the Mariana Islands.