Santa Rosalia, a steel-hulled screw freighter built in 1911 by William Hampton and Company, Ltd., Port Glasgow, Scotland, was acquired by the Navy at New Orleans on 20 May 1918 under United States Shipping Board (USSB) charter from the United States Steel Products Co.; and commissioned the same day.
Assigned to the Naval Overseas Transportation Service, the ship sailed for Hampton Roads on 1 June where she loaded general Army supplies. Sailing in convoy for France on the 19th, she reached Brest on 9 July and proceeded to Gironde to discharge her cargo. She got underway for home on 3 August and arrived at Baltimore on the 20th. Six days later, she headed for New York for repairs before sailing from Norfolk in convoy for Europe early in October. She reached Brest on the 28th and proceeded to St. Nazaire. On 11 November, while Santa Rosalia was unloading her cargo there, the armistice was signed, ending the fighting.
The ship got underway for Baltimore on 14 November and arrived there on 5 December. After transferring from an Army to a USSB account, the ship proceeded to New York to take on a cargo of general supplies. She stood out to sea on 19 January 1919, stopped at St. Thomas from the 25th through the 27th, and arrived at Montevideo, Uruguay, on 18 February. After unloading there, she took on a cargo and returned to New York. She again returned to New York, from a similar round-trip voyage to Montevideo on 6 June; decommissioned there on the 26th; and was transferred to the USSB the same day for simultaneous return to her owner.