"Panther" was pressed into service immediately after commissioning on the outbreak of the Spanish–American War, and was hastily converted into a troop transport. On 27 April 1898 the ship took aboard the First Marine battalion commanded by Lieutenant Colonel Robert W. Huntington in preparation for combat operations in Cuba, sailing from Brooklyn, New York just five days later. As a converted 'banana boat', the "Panther" proved totally inadequate as a troop transport, and the morale of the Marines suffered in the tight and sweaty confines of the hastily converted ship as it steamed southward. As the "Panther" was unarmed, it was forced to stop at Hampton Roads, Virginia to obtained an armed escort before steaming to Key West, Florida, to await orders while the Marines disembarked for battalion drills, rifle practice, and route marches. The ship then joined the North Atlantic Fleet, portions of which blockaded Cuba throughout the conflict. Admiral William T. Sampson received word from Key West early in May that Spanish Admiral Pascual Cervera y Topete's squadron was very likely to sortie against U.S. naval forces at Santiago, Cuba. "Panther" and other ships of the American fleet patrolled off that port until she steamed to Guantánamo with a battalion of 633 enlisted Marines and 24 officers commanded by Lt. Col. Robert W. Huntington.