King joined the crew of Samuel "Black Sam" Bellamy while still a juvenile, and is the youngest known pirate on record. While sailing on the sloop Mary Anne on November 9, 1716, Bellamy and his crew attacked and captured the Antiguan sloop Bonetta, which was then en route from Antigua to Jamaica. John King, then aged between eight and eleven, was a passenger. According to Abijah Savage, the Bonetta's commander, the pirates looted the ship, during which time King demanded to join Bellamy's crew. "Far from being forced to join," Savage wrote in his report, "John declared he would kill himself if he was restrained, and even threatened his mother, who was then on board as a passenger and his father who did not like him." After an initial show of defiance, Bellamy allowed King to join him. In the subsequent months, Bellamy and his crew would capture and loot many ships. In February 1717, they captured the Whydah, a heavily armed slave galley which Bellamy claimed for his flagship. After pillaging more ships, they sailed north and on April 26, 1717, the Whydah now overloaded, was wrecked in a strong storm off the coast of Cape Cod, killing Bellamy and most of his crew, including King.
King's remains were tentatively identified in 2006, when Barry Clifford, principal of Expedition Whydah Sea Lab & Learning Center in Provincetown Massachusetts, and Project Historian Ken Kinkor had partial human remains recovered from the Whydah wreck site analyzed by researchers at the Smithsonian Institution and Center for Historical Archaeology in Florida. The remains, consisting of an 11-inch fibula encased in a shoe and silk stocking, were determined not to belong to a small man, as originally thought, but to a young boy of King's approximate age. During excavation of the wreck site it was determined that King died because one of the cannon had pinned him to the seabed. While teenage pirates were common in the 18th century, and though the Royal Navy employed young boys as "powder monkeys" to carry gunpowder from ship's magazine to their cannons, children of King's age were unknown as pirates, giving him a unique place in the history of piracy. His brief time as a pirate lasted for just five months.