[1] Until approximately 1717/8, he was a merchant in New York City, subsequently relocating to Perth Amboy, New Jersey.
[2] Johnston represented the City of Perth Amboy with his father in the ninth New Jersey General Assembly (1727–1729 Legislative Session).
(1740–1744)[3] After his service in the General Assembly, on June 19, 1745, Johnston was appointed a member of the New Jersey Provincial Council, where he would serve until his death.
[2] In 1702, the royal instructions to Governor Viscount Cornbury named the Town of Perth Amboy as a constituency, apportioned two members to the New Jersey General Assembly.
[5] In 1718, Perth Amboy was granted city status; the same apportionment continued until the adoption of the New Jersey Constitution of 1776, which apportioned the entire New Jersey Legislature by county, thereby abolishing separate representation for cities.