Boswell met with Cairnie a few times in the fall of 1762. On October 26 of that year, he described him as"a man of great natural quickness and universal knowledge, particularly of all the little arts of life. He is shrewd, penetrating, and disposed to recommend himself by being serviceable. He is excellently formed for active life, and his manner of talking upon that subject gave me spirit".

Cairnie is again mentioned at the very beginning of Boswell's London Journal (November 15, 1762) because of an anecdote about France which he apparently told Boswell. A confidant of Boswell's, he was also one of the only people who knew about his affair and child with Peggy Doig. Boswell and Cairnie possibly met each other upon Boswell's initiation as Junior Warden in the masonic Canongate Kilwinning Lodge in December 1761; Cairnie became treasurer of the lodge a few months later.