Mendocino was named for a county on the northern California coast. Ingalls laid her down at Pascagoula on 20 September 1943 under a Maritime Commission contract and launched her on 11 February 1944. Mrs. William Tillotson sponsored her. She was a Bayfield-class attack transport. The Navy acquired her on 23 May 1944 and held her in ferry commission through 3 June so she could shift to Bethlehem Steel at Hoboken for conversion into an attack transport. Captain Walton R. Read commissioned her at Hoboken on 31 October 1944.
Chesapeake Bay shakedown came first. She embarked 705 naval passengers and left Norfolk for the Pacific on 10 December, reaching San Pedro, California, on Christmas Day. She went out to Pearl Harbor and came home again. On 5 February 1945 she departed San Francisco Bay for the Philippines, entered Leyte Gulf on 28 February, and through much of March trained for Okinawa. Transport Squadron 14 flew its flag in her. On 27 March she went out with the Southern Attack Force and was off the island early on 1 April. Beach White I, on the Hagushi shore, is where the 96th Infantry Division left her boats. Support gear followed, the off-load lasting through 6 April. Army wounded came aboard for the trip to Saipan. She then steamed to Pearl Harbor and arrived on 22 April.
As flagship of Transport Squadron 19 she trained in Hawaiian waters, returned to San Francisco on 4 June, and there embarked troops and loaded cargo. The Guam passage opened on 29 June and closed on 16 July. Men who had already fought in the Pacific then rode her home, San Francisco on 4 August. After the fighting stopped, an Army draft of 1,523 officers and men went out with her to the Philippines. Occupation troops embarked at Legaspi, on Luzon, and on 4 October she sailed in convoy for Japan. Yokohama on 13 October. Troops and cargo went ashore. Magic Carpet duty began after three days alongside. Veterans homeward bound filled the lift that ran 26 October to 8 November. A second troop lift finished on 27 December. San Francisco on 14 January 1946 was the start of the last passage. New York came on 1 February.
Decommissioning at New York was on 27 February 1946, and she was returned to the War Shipping Administration the same day. Her name left the Navy list on 12 March 1946. Pope & Talbot, Inc., of San Francisco, bought her and renamed her P & T. Seafarer. One battle star was awarded.