Cynthia's widowed mother was a librarian in a small Indiana town.
He was educated at the university and was librarian from 1697 to 1701.
He also held the offices of librarian of the Bibliotheque du Roi, and of perpetual secretary of the Academie des Inscriptions.
In January 1757 he succeeded David Hume as librarian to the faculty of advocates, but soon relinquished this office on becoming tutor in the family of Lord Bute.
In order to improve his financial position, he accepted early in 1786 the post of librarian to the elector-archbishop of Mainz, who bestowed many important offices upon him and obtained his elevation to nobility from the emperor in 1791.