She was laid down under Maritime Commission contract (MCV hull 40) on 29 June 1944 by the California Shipbuilding Corporation, Wilmington, California; launched on 4 September 1944; sponsored by Mrs. Arthur S. Tode; acquired by the Navy on loan charter and delivered on 14 November 1944; and commissioned on 15 November 1944.

Assigned to Transport Squadron 17 (TransRon 17) following shakedown, St. Mary's departed Los Angeles on 1 January 1945; loaded bulldozers, airplane engines, bomb service trucks, and other equipment at San Diego; and, on the 4th, sailed for Manus, Admiralty Islands.

Arriving in Seeadler Harbor on the 21st, she offloaded her cargo and steamed to Humboldt Bay, New Guinea, whence she carried troops to Leyte, 31 January to 6 February.

on the 19th, she moved around to Okinawa's southern coast for a diversionary landing; then returned to Hagushi to discharge the remainder of her cargo and troops.

St. Mary's embarked occupation troops and sailed for Japan, arriving in Tokyo Bay on 2 September, just prior to the signing of the official surrender documents.

In 1956 St. Mary's was withdrawn from the Reserve Fleet as part of a Repair Program, GAA-Pacific Far East Lines, and then returned.