He attended Liberty Hall Academy and later was a trustee of Washington College. Johnston was a prosperous farmer by the time the American Revolution began. In 1776, he was appointed a captain in the county militia. In 1781, he participated in the Virginia campaign which led to Lord Cornwallis' surrender. Representing Augusta County in the House of Delegates from 1778 to 1791, Johnston was chair of the House committee on religion and helped pass the "act for establishing religious freedom" in 1786.
