The king who was known as the king who lost America is King George III (d). He reigned from 1760 until 1820. He reigned for one of the longest periods of time by any British Royal in History. He is primarily remembered as the king who lost the American colonies as well as the king who went mad. Even so, he reigned until his death in 1820.The American Revolution was primarily sparked by the lack of American representation in the British parliament.