The eldest son of James Keith, he was born in the Old House of Aquhorsk in Marr, near Aberdeen, on 6 November 1752. He took his degree from Marischal College and the University of Aberdeen in 1770, was licensed by the presbytery of Aberdeen on 14 July 1774. Keith was presented by the commissioners for George Keith, 10th Earl Marischal, 9 May 1776, to the living of Keith-Hall and Kinkell, Caskieben, Aberdeen. A legal difficulty arose when, the following day, the Earl Marischal himself, then resident in Potsdam, gave a presentation to Thomas Tait, minister of Old Machar. After legal proceedings in the church courts and the court of session, the case was finally decided in Keith's favour by the House of Lords in April 1778 and he was ordained to the living on 14 May 1778.
The eldest son of James Keith, he was born in the Old House of Aquhorsk in Marr, near Aberdeen, on 6 November 1752.