Biography
Son of Sir James Johnstone (1697-1772), 3rd Bart of Westerhall, Provost of Lochmaben. In 1763 Johnstone became the first British governor of British West Florida, but he was removed by King George III in 1767. MP for Cockermouth and Appleby at some time after 1767.
Life with Boswell
Boswell was in the company of a Captain Johnstone of the Navy, son to Sir James on January 5, 1763, at Lord Eglinton's, together with Sir James Macdonald, and Thomas Miller, the Lord Advocate of Scotland.