He was born Alexander Muir in Perthshire on 2 March 1764 the son of George Muir of Cassencarrie House, in Kirkmabreck near Creetown[1] and his wife, the Hon Margaret MacKenzie of Delvine.
[2] He trained in Law and passed the Scottish bar as an advocate in 1788.
In 1793 he was elected a Fellow of the Royal Society of Edinburgh.
His proposers were Dugald Stewart, Dr James Gregory and Alexander Fraser Tytler, Lord Woodhouselee.
[4] His only son, and successor was Sir John William Pitt Muir-MacKenzie (1806–1855),[5] named in deference to William Pitt the Younger, the then prime minister.