Laid down on 24 August 1944 by Boston Navy Yard, and launched on 17 November 1944 as APB 39, sponsored by Mrs. Lillian Gaudette, the ship was named Mercer, after counties in eight states, on 14 March 1945, and commissioned on 19 September 1945.

[1] Mercer, a barracks ship built on an LST hull, commenced her career of berthing and messing large numbers of naval personnel at New York on 7 November 1945.

[1] Returning to the United States on 21 July, she underwent an overhaul and reported to Davisville, Rhode Island, for duty as a barracks ship for Mobile Construction Battalions.

Her Seabee duty was interrupted when she sailed to Washington, D.C., to berth midshipmen of the United States Merchant Marine Academy taking part in President Dwight D. Eisenhower's 1953 Inaugural Parade.

[4] IX 502 served during the 1980s as a berthing barge at Naval Station San Diego, providing sleeping quarters, messing, and classroom accommodation for crews of ships undergoing maintenance.