[1] An associate of George Scot of Pitlochie, he was a druggist from Edinburgh and emigrated to the Thirteen Colonies in 1685 aboard the Henry and Francis.
Between 1710 and 1714 Johnstone represented Perth Amboy in the New Jersey General Assembly.
He was first recommended to the Crown for the New York Provincial Council by Governor Robert Hunter in 1715 to fill a vacancy caused by the death of Dr. Samuel Staats.
[4] Johnstone was finally appointed in 1720, but by 1722 Hunter's successor, William Burnet, complained that Dr. Johnstone had "without any leave obtained under the Hand and seal of any Governor or president, now resided for above two years last past in New Jersey", and asked for his removal from the Council.
John Johnstone had returned to the New Jersey General Assembly in 1721, again representing Perth Amboy.