Plant collector and horticulturist. McNab was born in the parish of Dailly (South Ayrshire), one of twelve children of a farmer. He became an apprentice gardener at Dalquharran and then gained a position with the Earl of Haddington at Tyninghame House (East Lothian). Soon after he was recommended to William Aiton (1731-93) as a gardener at the Royal Botanic Gardens at Kew (London), where he arrived in 1801. There he was noticed by King George III, who had established Kew Gardens and took a keen interest in broadening its plant collection. McNab received plants from around the globe including those captured from the French which had been intended for a Botanic Garden in Paris. In 1810, Prof. Daniel Rutherford (1749 - 1819), Regius Keeper of the Royal Botanic Garden Edinburgh, asked Sir Joseph Banks to recommend a suitable person to serve as Superintendent of the Garden in Edinburgh, then located on Leith Walk, and McNab took the position despite a considerably lower salary. As the collection increased, Rutherford and McNab pressed for more space and land was acquired at Inverleith. McNab was responsible for moving existing plants and trees to the new location between 1820 and 1823.
Like he had at Kew, McNab received plants from around the world, but he became a particular expert in the cultivation of ericaceous plants from Southern Africa and published a treatise on their cultivation in 1832. He also maintained an interest in the native flora of Scotland, taking excursions into the Highlands and producing watercolour illustrations of live specimens before pressing them for the herbarium. McNab also supplied other botanic gardens with specimens he had cultivated.
He died working in the garden he loved and was buried in Old Calton Burial Ground. His son, James McNab (1810-78), succeeded him in the Royal Botanic Garden Edinburgh. A genus of ericaceous plants was named Macnabia in his honour and he is also remembered at the Scottish Plant Hunter's Garden in Pitlochry.
