The USS Barbarossa, a steel hulled, twin screw steamer built for the North German Lloyd Steamship Co. was launched at the Hamburg, Germany, yard of her builders, Blohm and Voss, on 5 September 1896 and carried freight and passengers in the New York and Australian trade into World War I. Unable to return to Germany due to the Allied blockade at the start of that conflict, she sought haven at Hoboken, New Jersey, where she was taken over by the Federal Government upon the entry of the United States into the war on 6 April 1917.
Anticipating American seizure, the ship's German crew had attempted to cripple her machinery with sledgehammers. Therefore, Barbarossa was towed to Brooklyn for repairs by the Robins Dry Dock and Repair Co., under supervision of the United States Shipping Board (USSB). Turned over to the Navy soon thereafter and assigned the Identification Number (Id. No.) 3012, Barbarossa was commissioned on 3 August 1917, Comdr. Harry L. Brinser in command.
The transport renamed Mercury on 6 September 1917 was put to sea for trials on 19 December with her newly repaired machinery. Returning to Hoboken on 21 December, she began loading cargo and stores for her initial voyage for the Navy. Her first contingent of troops arrived on 3 January 1918, and she sailed from New York the following day in a convoy escorted by Seattle (Armored Cruiser No. 11). Mercury later rendezvoused with a screen of destroyers sent from Queenstown and reached the mouth of the Loire River on 16 January.
As the ship approached St. Nazaire, her lookouts sighted a German submarine some 2,000 yards off her port beam. The U boat, on a converging course, miscalculated her trim at periscope depth while bringing her torpedo tubes to bear. This caused the U boat to broach, betraying her presence. The escorting destroyer attacked the enemy craft and drove her off. Departing St. Nazaire in ballast on 27 January, Mercury ran into such heavy weather on the return voyage that she was obliged to put in at the Azores to take on coal before finally returning to New York on 15 February.
Before the armistice stilled the guns on the western front, Mercury made seven more round trip voyages carrying 18,542 troops to France, as well as thousands of tons of supplies. Embarking troops at Hoboken or Newport News, Va., she disembarked them at such ports as St. Nazaire, Brest, and Paulliac. The end of hostilities found her at Newport News.
With the end of the war, the tide of troops was reversed, as the doughboys and marines returned from "over there." Mercury and the other transports which had performed such yeoman service in speeding men eastward to France now found themselves engaged in the reverse movement. Eight times, Mercury plied the Atlantic, bringing home 20,871 men. She returned to New York from her last voyage on 19 September 1919. Transferred to the Army Transport Service simultaneously with her decommissioning on 27 September 1919, she briefly served as an Army transport before being turned over to the War Shipping Board for scrapping in 1924.