He represented the city of St. John from 1820 to 1830 and Queen's County from 1834 to 1842 and from 1847 to 1850 in the Legislative Assembly of New Brunswick.
He was born near Gagetown, New Brunswick, the son of Hugh Johnston and Ann Gilzean.
Johnston married Harriet Maria, the daughter of Thomas Millidge in 1829.
Johnston resigned from the Executive Council in 1845 after William MacBean George Colebrooke named his son-in-law provincial secretary.
Johnston was persuaded to return to office in 1846 by the colonial secretary and served until 1848.