Mission San Antonio was laid down on 15 January 1944 under a United States Maritime Commission contract by Marinship Ship Corporation, Sausalito, California; launched 8 April 1944; sponsored by Mrs. Marian McClure; and delivered 24 May 1944.

She served with MSTS until 22 November 1954 when she was returned to the Maritime Administration (MARAD) and laid up in the National Defense Reserve Fleet at James River, Virginia.

She was resold to Hudson Waterways Corporation on 24 January 1966 for conversion into a container ship by Savannah Machine and Foundry and renamed Seatrain San Juan, IMO 6519716, in September 1966.

Upon completion of the conversion in 1967 she began hauling containerized cargo for Seatrain Lines between the east and west coasts of the United States and to Puerto Rico.

The ship was sold on 30 June 1972, to Tyler Tanker Corp. She was returned to MARAD on 20 November 1973, and was laid up in the National Defense Reserve Fleet, James River Group, Fort Eustis, Virginia.