Johnston was born c. 1730, near Perth Amboy, New Jersey, the son of George Johnston and Bathsheba Lucas. His paternal grandfather emigrated from Scotland. Johnston's father died when he was young and his mother remarried to Matthew Robinson.
Johnston was educated in the colony of New York. Thence he removed as a young man to Newport, Rhode Island, where he was admitted a voter on April 30, 1751. He studied law with his step-father, Matthew Robinson, a lawyer of reputation and a wide reader, whose large private library was augmented by that of Johnston's maternal grandfather, a Huguenot named Lucas, who lived for a time in Newport.
Johnston attracted notice by marked ability in his profession and an impressive self-confidence. He was appointed in 1754 and in 1756 to help in preparing bills for the General Assembly. In October 1756 he was made a first lieutenant in a regiment to be sent against Crown Point. In June 1757 he was appointed attorney-general, the candidate elected having died, and was re-elected each year until May 1766, serving practically nine years, though after the Stamp-Act riots of 1765 his name was omitted from a committee to revise the laws, and another attorney was appointed to join him in carrying on a suit brought by the colony against a late collector of customs. During his term of office, in 1760 he was one of four to revise the laws, and in 1763, one of four to draw up regulations for a hospital for smallpox inoculation and recommend a place to build it. The town of Johnston, separated from Providence in 1759, is said to have been named for the attorney-general.
On August 27, 1765, Johnston, stamp-distributor, and two others who had supported the rights of Parliament were hanged in effigy; on August 28, a reckless mob did serious damage to their houses and furniture. All three fled for their lives to the armed ship Cygnet in the harbor, and when Johnston came ashore next day he was forced to sign a paper agreeing not to execute his office of distributor without the consent of the colonists. In a letter to the collectors dated November 22, 1765, he maintains that before the riots "no application was ever made to me by any one person to resign said office"; but regard for life and property, he asserts, obliged him to deposit the stamped papers for safe keeping on the Cygnet, when they came, and hence he could not supply the collectors' demand for them. He was evasive when the Governor pressed him to answer whether he was or was not going to distribute stamps; but the Governor wrote to England that Johnston had resigned. The other two sufferers in the riots went to England and presented exaggerated accounts of their losses; Johnston also made an unwarranted estimate of his, at first. The matter called forth a prolonged correspondence between the Treasury and the General Assembly of Rhode Island, since it happened that the military disbursements of 1756 had never been repaid to the colony, and the Treasury refused payment until the three persecuted Loyalists were compensated. Although the claims were moderated, and the Assembly, after severe revision, allowed them, subject to payment of their own claim upon England, the matter was still under discussion in August 1773, and was never settled.
Johnston remained unmolested in Newport, but on July 18, 1776, having refused the test of allegiance, he was ordered interned at South Kingstown. He held civil appointments at Newport during the British occupation, and left for New York when the town was evacuated in 1779. His property was confiscated, and, in spite of a pension from the British government, he died insolvent, to the distress of his step-father, who, having been Johnston's surety, was obliged in extreme old age to go to court and defend suits which were brought against him.
After his death, Johnston left a widow and four children.
