Sibley was born in Sutton, Massachusetts, the son of Ruth and Reuben Sibley. After completing preparatory studies, he graduated from the College of Rhode Island and Providence Plantations (now Brown University) at Providence in 1794. He studied law, was admitted to the Bar in 1795 and began a practice in Marietta, Ohio, which was then part of the Northwest Territory. He soon moved to Cincinnati and then moved again to Detroit, Michigan in 1797, shortly after the British handed over the fort in 1796. When he arrived, Sibley was one of only two lawyers in Detroit. Being a pioneer lawyer was a physically challenging profession, often requiring long travel by horseback through wilderness over Indian trails in all types of weather to attend the territorial courts in Cincinnati, Marietta, or Chillicothe, Ohio.