On March 22, 1804, President Thomas Jefferson nominated Johnson to be an Associate Justice of the United States Supreme Court, as the successor of Alfred Moore. He was confirmed by the United States Senate by voice vote on March 24, 1804, and received his commission the same day. Jefferson nominated the 32-year-old Johnson because he was from a similar geographic region as Moore, because Johnson's health would allow him to travel regularly (a requirement of the position at the time), and because Johnson adhered to republican principles. He was the first member of the Court who was not a Federalist.