Both El Capitan and sister ship El Almirante were designed by the line's engineer, A. S. Hebble, as single screw ships with three complete steel decks on a transverse framing system. Watertight bulkheads extending to the main deck were fitted at the forepeak, between #1 and #2 holds, aft the engine room and after peak. A watertight bulkhead extending only to the lower deck was fitted aft the ballast tank.
Propulsion was by a triple-expansion steam engine of 24.25inches, 41.5inches and 72inches diameter with 48inches stroke designed for 75 revolutions per minute with steam provided by two oil burning Scotch marine boiler designed to allow the ship to make 11kn when fully loaded to the designed 25feet draft. The ships were specifically designed with a limited speed of 12kn as their 6,500-ton cargo was to be "slow moving freight."
The precise launch date is given on widely differing dates in sources. 18 August 1917 is given by Navy. September 21 is given in the Annual Report of the Commissioner of Navigation to the Secretary of Commerce for the fiscal year ending 30 June 1918. The photograph of the ship underway that leads the article "New Southern Pacific Freighters" has an annotation of "8-4-17" indicating the ship was launched, fitted out and underway by 4 August 1917.
Before delivery to the Southern Pacific Company the ship was requisitioned by an order on 3 August 1917 directly from the builder by the USSB Emergency Fleet Corporation with payment of $230,000. The 19 September 1917 bill of sale specifies that "nothing herein shall be construed to the prejudice of any legal or equitable right or claim" by the "former owner," the Southern Pacific Company. After the United States entered World War I, the United States Shipping Board transferred El Capitan to the U.S. Navy for war service on 21 March 1918. The Navy assigned her the naval registry Identification Number (Id. No.) 1407 and commissioned her the same day as USS El Capitan.
Assigned to the Naval Overseas Transportation Service, El Capitan made four transatlantic voyages between 29 March 1918 and 23 November 1918. She carried various supplies and equipment from United States East Coast ports to Brest, Le Verdon-sur-Mer, and Le Havre in France, and to Plymouth and Devonport in England. On her second voyage, one day out of Philadelphia, Pennsylvania, she sighted a German submarine abeam. Her guns forced the submarine to dive before it could attack.
El Capitan was decommissioned on 1 February 1919 and transferred to the Shipping Board the same day for return to the Southern Pacific Steamship Company for commercial service. On 30 September 1921 Captain J. H. Halsey, the ship's wartime commander now in command in civilian service, hosted a retirement luncheon for the line's Commodore, Edwin F. Parker.
Before the United States entered World War II, El Capitan was taken over by the United States War Shipping Administration (WSA) and was placed under the operation of United States Lines as WSA's agent on 26 June 1941. She was transferred to Panamanian registry (a flag of convenience) on 1 October 1941 without an official number and change in code letters.
El Capitan was part of Arctic convoy PQ 17, dispersed on Admiralty orders on 4 July 1942, and after picking up 19 survivors of reached Novaya Zemlya where a small convoy of five other merchant ships and escorts was assembled. That convoy was attacked on 9 July 1942 by Ju 88 aircraft with El Capitan suffering ruptured hull with holds #4 and #5 and the engine room flooding. All personnel, including those rescued, were taken off by, an armed trawler which also tried to scuttle the ship, and made Archangel. The hulk was torpedoed by U-251 just after midnight on 10 July.