He practised for many years in Worcester 'with nigh reputation and success,' and he gained many learned and influential friends, including L. M. da Costa [q. v.] and Lady Mary Wortley Montagu.
In 1745 he was consulted, together with Philip Doddrige by Isaac Maddox, then bishop of Worcester, respecting the foundation of Worcester Infirmary, and he was attending physician at that institution from its establishment until his retirement from practice in 1751, when he settled in Kidderminster.
The bishop wrote him an affectionate letter as a stimulus 'to usefulness, even in retirement,' and in 1758 he responded by producing 'The History of Health and the Art of Preserving it,' Edinburgh, 8vo, dedicated to the bishop, commencing with a succinct account of man's food before the fall, and containing summaries of the general rules of health laid down by eminent physicians from Moses onwards.
A third edition appeared also at Edinburgh in 1760, bearing fruits of Mackenzie's friendship with the Wortley Montagus in the shape of an appendix, containing 'A Short and Clear Account of the Commencement, Progress, Utility, and Proper Management of Inoculating the Small Pox as a valuable branch of Prophylaxis.'
Mackenzie also wrote 'Essays and Meditations on Various Subjects,' a pious volume published posthumously at Edinburgh in 1762, and he contributed 'History of a Complete Luxation of the Thigh' to 'Essays and Observations, Physical and Literary' (1756, ii.