Johnston served as Governor of North Carolina from 1787 to 1789. He presided over both conventions called to ratify the U.S. Constitution. The first in 1788 rejected the Constitution in spite of Johnston's strong support. He called another convention in 1789 which did complete ratification. After statehood Johnston resigned as governor to become one of the state's first two United States Senators, serving from 1789 until 1793. In 1800 he was made a Judge in the Superior Court of North Carolina, an office he held until his retirement in 1803.