Born at Uras in Kincardineshire, Scotland, on 7 February 1681, he was the second son of Alexander Keith and Marjory Keith (née Arbuthnot). He was educated at Marischal College, Aberdeen between 1695 and 1699; graduating with an A.M. in about 1700.
The grave of Bishop Robert Keith, Canongate Kirkyard, Edinburgh
In 1716, he and other clergy in Edinburgh were prosecuted by the Commission of the Justiciary for not praying for King George I. He was prohibited from the ministry and fined.
In 1719, he married Isobel Cameron, daughter of the Reverend John Cameron, Incumbent of Kincardine-in-Menteith. They had two children: an unnamed daughter (died before 1757), and Catharine who married Stewart Carmichael, an Edinburgh merchant, in 1752.
In March 1720, he was one of the clergy of Edinburgh who met to elect Arthur Ross's successor.
He was deeply versed in archeology, numismatics and Scottish antiquities. His published works include History of the Affairs of Church and State in Scotland and An Historical Catalogue of the Bishops of Scotland.
He died in Leith on 20 January 1757, aged 75, and is buried in Canongate Churchyard on the Royal Mile in Edinburgh. The grave lies slightly west of the church.