After being educated at the parish school he was trained as a gardener, and having filled several posts in Scotland went to Ireland in 1803.
[1] He visited the west of the island in 1804 and 1805, and as a result published a 'Catalogue of the Rarer Plants of Ireland' in the Transactions of the Royal Dublin Society for the following year.
[1] The governors of Trinity College, Dublin, having determined to establish a botanical garden, Mackay was recommended to them as a curator, and he held the post from 1806 until his death.
Soon after his appointment he was elected an associate of the Linnean Society, and in 1850 the university of Dublin bestowed upon him the degree of LL.D.
[1] Mackay discovered several species of plants new to the British Isles, and contributed largely to Sir J. E. Smith's English Botany (1790–1814).