He was the son of John Sinclair, 3rd Earl of Caithness and Elizabeth, daughter of William Sutherland, 5th of Duffus.

According to Roland Saint-Clair, the earl was required to bring his countrymen with him to the court which he either neglected or declined to do and as a result he was imprisoned at Inverness, Aberdeen and Edinburgh successively, and was not released until he had paid a considerable sum of money.

[2] On 18 December 1556 George, Earl of Caithness received a remission under the Great Seal of Scotland for oppression of the lieges.

[1] On the night of the murder of David Rizzio on 9 March 1566, the Earl of Caithness was with the party who tried to rescue the Queen, but being outnumbered retired from the contest.

Two days later the Queen escaped to Dunbar Castle with Lord Darnely where they were joined by their friends including the Earl of Caithness.