In 1819 he printed anonymously at Vevay for private distribution a 'Sketch of a New Theory of Man,' which was translated into French, and reprinted in English at Bath, 1822.

On his return to England, after passing several winters at Bath, he removed to Chichester, where he died 29 January 1831 at the age of eighty-three.

He married in 1784 the daughter of a French clergyman, who translated into English the 'Letters of Madame de Sévigné,' published in London, 12 mo, 8 vols.

Mackie was a religious man and an attached member of the church of England, notwithstanding his Scottish parentage and education.

A fine portrait of him was painted in miniature by George Engleheart in 1784 (the date of his marriage); another by Marchmont Moore in 1830 was engraved by Samuel Freeman in the same year, and a drawing in water-colours was executed by Slater in 1808.