The following day, she was taken over by the United States Maritime Commission and transferred to the Navy. Commissioned 27 April 1942 with Lieutenant Thomas G. Warfield in command, she was renamed Rochambeau and designated AP-63 on the 29th. Rochambeau, converted for use as a casualty evacuation ship, departed Oakland, California on 20 October for her first operation, under the U.S. flag. With replacements and reinforcements for the Guadalcanal campaign embarked on her westward passage, she made Nouméa; disembarked her passengers; replaced them with casualties from hospitals there, at Suva, and at Bora Bora; and returned to San Francisco on 3 December. At the end of December, she sailed west again. Extending her range to New Zealand and Australia on that voyage, she limited her next run, 9 to 27 March, to New Caledonia and the New Hebrides. On that trip she carried Lieutenant John F. Kennedy to Espiritu Santo where he was transferred to LST-449 and taken to the Solomons. During May, Rochambeau remained in waters off California, then, on 5 June, resumed her passenger/casualty runs to the south and southwest Pacific. Continuing those runs well into 1944, she added ports in New Guinea to her stops in September 1943 and the central Solomons in the spring of 1944. On her last run, 16 November 1944 – 17 January 1945 she brought back casualties from hospitals on Eniwetok, Guam, and Kwajalein in the company of the and.
Decommissioning
On 9 February, Rochambeau headed for New York City. Arriving on the 25th, she was decommissioned and transferred to the U.S. Maritime Commission's War Shipping Administration on 17 March. Her name was struck from the Naval Vessel Register at the end of the month. Then she was returned to her French owners Messageries Maritimes and resumed the name Marechal Joffre and, operating for WSA, was used to transport American troops from Europe to the United States. The hull was repainted black with a white superstructure and , Egypt, in 1951 with the rebuilt funnel From 1946 to 1950 she served as a troopship for the French Army in Indochina. Between 1950 and 1951 she returned to La Ciotat for rebuilding and the two funnels were replaced with a single oval funnel and painted white overall. She then served as a liner across the Indian Ocean to the Far East. She was scrapped in Osaka, Japan, in 1960.