Johnston was admitted to the bar and began the practice of law in Edenton. In 1759 he was elected to the Province of North Carolina House of Burgesses and would serve in that body until it was displaced in 1775 as a part of the Revolution. As a strong supporter of independence, he was also elected as a delegate to the first four provincial congresses and presided over the Third and Fourth congresses in 1775 and 1776. In the time after the Royal Governor Josiah Martin abdicated in 1775, he was the highest-ranking official in the state, until Richard Caswell was elected president of the Fifth Provincial Congress.