He was a member of the Privy Council of Scotland and Lord Lieutenant of the county of Caithness.

[4] George Sinclair, 6th Earl of Caithness was present with Archibald Campbell, 1st Marquess of Argyll when Oliver Cromwell was proclaimed "Chiefe Magistrate of thir thrie nations, ouer the Crose of Elb".

[2] George Sinclair, 6th Earl of Caithness married Mary, daughter of Archibald Campbell, 1st Marquess of Argyll, but had no children.

It was recorded on 3 May 1676 that "Mr Andrew Munro, minister of Thurso, did represent that the Earl of Caithness, being visited with heavie sickness, did earnestlie desire that all the Brethen of the Presbie should remember him in the publick and private prayers to God, which desire was cordially entertained".

Inflamed with drink, the Sinclairs attacked the force of Campbells and were routed in what is known as the Battle of Altimarlach.