King was born in New York City into a poor Irish family. He was indentured as an apprentice to a jeweler in Manhattan at the age of nine. At the age of 11 he fled the city and stowed away on a ship bound for Mobile, Alabama. Upon discovery, he was adopted by the crew and trained in navigation, becoming a steamboat pilot by the age of sixteen. By 1850 he was a steamboat captain and had formed a steamboat firm with three other partners. This firm achieved “nearly monopolistic” control on the Rio Grande for most of the years between 1850 and 1874, when the partnership dissolved.