He wrote one successful book, Chrysal, or the Adventures of a Guinea, a somewhat sombre satire.
Born at Carrigogunnell, County Limerick about 1719, he was educated at Trinity College, Dublin, but is not known to have taken a degree.
He was called to the bar, but extreme deafness prevented his practice except as a chamber lawyer, where he did not succeed.
[1] Johnstone's major work, entitled Chrysal, or the Adventures of a Guinea, and frequently reprinted, appeared in 4 vols., London, 1760–65.
The book, a succès de scandale, claimed to reveal political secrets, and to expose the profligacy of well-known public characters.