After two years of active service on "Potomac" and "San Jacinto", Herndon was assigned in 1855 as commander of the Atlantic Mail Steamship Company steamer SS "Central America", on the New York to Aspinwall, Panama, run. Navy captains were assigned to command the mail steamers on the Atlantic and Pacific runs; the ships were operated and maintained by companies under contract to the federal government. At the time, such mail steamers transported large quantities of gold from the California gold fields to cities on the East Coast and the US Mint in Philadelphia. ("Central America" had recently been renamed from "George Law". Aspinwall was an English name for Colón, Panama.)