Keith served in Flanders under the Duke of Marlborough from 1708 to 1711, but for considering placing the Old Pretender on the British throne on Queen Anne's death he was deprived of his commission (or resigned).

[4] He retired from the Spanish court in 1741, finding it impotent to help the Jacobite cause and warning the Old Pretender not to trust the more positive reports from his agent in Paris, Lord Sempill.

[5] He then served Frederick the Great as Prussian ambassador to Spain from 1759 to 1761, informing the British government of Spanish preparations to enter the war on France's side, which gained him his pardon by George II on 29 May 1759.

Frederick appointed Keith his ambassador to Great Britain in 1759, but despite brief return trips to Scotland in 1761 and 1763–64, he found the climate and his neighbours unfavourable.

At Frederick's invitation, Keith sold the estates and returned to Prussia for good, becoming a close friend of the Prussian king and dying in Potsdam in 1778.