King was born in Launceston, Cornwall. His father was a draper, selling cloth and fabrics. He joined the navy in 1770 at the age of 12. He was made the captain's servant on the "HMS Swallow". He served in the West Indies and then fought in the American War of Independence. He became a midshipman and later a lieutenant. He served with Captain Arthur Phillip in "HMS Ariadne". He went to India with Phillip in 1783 in the ship "HMS Europe". Phillip was very pleased with King. He made King a lieutenant on HMS "Sirius" when he took the First Fleet to Australia in 1787 to start the penal colony.