Churchman, historian and Jacobite. Born in Uras (then Kincardineshire), Keith belonged to the family of the Earls Marischal. He was educated in Aberdeen. He worked as a tutor (1703-10) and was then admitted as a Deacon to the Scottish Episcopal Church. After serving as chaplain to the Earl of Errol and accompanying him to Europe, Keith settled in Edinburgh, where he served as a minister. He was ordained as Bishop of Caithness, Orkney and the Isles in 1727 and became Bishop of Fife in 1733. A staunch Jacobite, he corresponded with Prince Charles Edward Stuart (1720-88) on ecclesiastical affairs.
Keith published his History of the Affairs of Church and State in Scotland in 1753, which dealt with the period of the Reformation, and also A Catalogue of Scottish Bishops (1755).
He died at his home, Bonnyhaugh House in Edinburgh, and was buried in the Canongate Kirkyard.