Born in Windsor, Connecticut, Ellsworth attended the College of New Jersey where he helped found the American Whig–Cliosophic Society. In 1777, he became the state attorney for Hartford County, Connecticut and was selected as a delegate to the Continental Congress, serving during the remainder of the American Revolutionary War. He served as a state judge during the 1780s and was selected as a delegate to the 1787 Philadelphia Convention, which produced the United States Constitution. While at the convention, Ellsworth played a role in fashioning the Connecticut Compromise between the more populous states and the less populous states. He also served on the Committee of Detail, which prepared the first draft of the Constitution, but he left the convention before signing the document.