Son of William Keith (d. 1712), 9th Earl Marischal, and Mary Drummond (1675-1729). Brother of James Francis Edward Keith (1696-1758), who was a Prussian Field Marshal and Governor of Berlin. One of his sisters, Anne Keith (d. 1728) was the first wife of the 6th Earl of Galloway.
Keith went into exile after having supported the Jacobite rebellion in 1715. He later gained a reputation throughout Europe for being an excellent and much-respected diplomat. A favourite of Frederick the Great of Prussia, Keith was appointed Prussian ambassador to Paris (1751) and Spain (1758), as well as Governor of Neuchatel (1752), when the city was under Prussian supremacy. His ambassadorship in Spain enabled him to warn the British about an impending Franco-Spanish alliance; as a result of this, he was granted a pardon, and his title and part of his former Scottish possessions were restored to him. In the early 1760s, he even acted as Prussian Ambassador to England.
In 1764, however, he was called back to Berlin, never to return to Scotland. He was reputedly one of the select few, who was always guaranteed one of the four guest rooms in Frederick the Great's famous but small Sans Souci residence near Potsdam.
George Keith was a long-time friend and protector of the philosopher Rousseau, who wrote of him in his Confessions:
"How many affectionate tears have I shed on my path as I thought of the goodness, the lovable virtues, the gentle philosophy of that venerable old man! I called him my father, he called me his child. These two names give some idea of the attachment which bound us together, but they fail to express the need which we had of each other, and of our unremitting desire to be in each other's company"1
James Boswell and George Keith were third cousins, both descending from John Erskine, 18th Earl of Mar (d. 1634) and Mary Stuart (d. 1644).
The first mention of Keith in the Yale Trade editions is in a letter from James' father Alexander Boswell, Lord Auchinleck (dated April 2, 1764) to James, who had communicated to his father a wish to visit some of the numerous German courts of the time. Alexander replied that Earl Marischal, who was in Scotland at the time, particularly recommended visits to the Prince of Brunswick, the Prince of Baden-Durlach and, of course, the King of Prussia, Frederick the Great. On June 4, Boswell received the news, that he was to accompany Keith through some of the courts on the latter's return trip from Scotland to Berlin. On June 13, Keith arrived in Utrecht, and a few days later he and Boswell set out on their journey, accompanied by Keith's adoptive daughter, Emet-Ulla.
The three travelling companions arrived at Potsdam on July 2, after having visited Brunswick on their way.
Boswell saw little of Keith in the ensuing months, but on September 5 he visited him in Potsdam together with Alexander Burnett. In the evening he played a slight joke on him together with Lauchlan MacPherson, which was taken with good humour, and on the 6th the Boswell and Keith had an intimate and friendly conversation, which lead to Keith's writing to Lord Auchinleck, asking him to allow Boswell to travel around Europe for another year. In the end, as Boswell wrote in his journal, "[h]e took down from his bookcase the history of Robert the Bruce in old verse, and made me a present of it, writing upon it, "Scotus Scoto," and saying, "Now you must read this once every year." I had almost cried before the good old man. We dined with his Lordship."
Boswell and Keith stayed on friendly terms and corresponded until Keith's death in 1778.