Johnson was fourteen in the summer of 1787 when delegates met at the Constitutional Convention. According to one Supreme Court historian, "nothing shaped Johnson’s habits of mind more powerfully than the experience of revolution." 1790, William Johnson graduated from Princeton University first in his class. Three years later, he passed the bar after tutelage under Charles Cotesworth Pinckney. Johnson was an adherent of the Democratic-Republican Party, and he represented Charleston in the South Carolina House of Representatives from 1794 to 1800. In his last term, from 1798 to 1800, he served as Speaker of the House. In 1799, Johnson helped pass a bill to reorganize the state judiciary. Later that year, he was also appointed an associate justice of the state Court of General Sessions and Common Pleas, a position created by Johnson's reorganization. The service on that court also included a position on the state Constitutional Court, the highest court in the state at the time.