The son of a prosperous Massachusetts merchant, King studied law before volunteering for the militia in the American Revolutionary War. He won election to the Massachusetts General Court in 1783 and to the Congress of the Confederation the following year. At the 1787 Philadelphia Convention, he emerged as a leading nationalist, calling for increased powers for the federal government. After the convention, King returned to Massachusetts, where he used his influence to help win ratification of the new Constitution. At the urging of Alexander Hamilton, he then abandoned his law practice and moved to New York City.