Son of John Johnston (d. 1749 or 1759) of Heithat (Later of Grange and Upperbanks). He was a writer in Edinburgh (a position resembling that of an English solicitor).
In early biographies of Boswell, including the 1951 Yale edition of Boswell in Holland, Johnston was thought to be of the same age as Boswell. However, an appendix to the Correspondence of James Boswell and John Johnston of Grange (1966) argues that Johnston was probably born in 1729 and certainly no later than 1731
Boswell's oldest friend, whom he probably met in Robert Hunter's Greek class at the University of Edinburgh in 1755. Their correspondence from September 1759 to Johnston's death in 1786 was published in 1966 as the first volume of the Yale research editions.
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The Correspondence of James Boswell and John Johnston of Grange was published by Yale University Press in 1966 and is available via AbeBooks.