Johnson was sworn in on January 3, 1820. The Senate was at that time grappling with the admission of the Missouri Territory and the Maine District (then part of Massachusetts) as states. When a bill was introduced which would bar slavery in the territories north and west of Missouri, Johnson was assigned to a select committee of five senators to consider it. The Maine and Missouri questions had been combined into one bill; Johnson voted against an amendment to separate them, which was defeated.