George Sinclair of Keiss was the son of Francis Sinclair of Northfield, who in turn was a younger son of George Sinclair, 5th Earl of Caithness. His first cousin-once-removed was George Sinclair, 6th Earl of Caithness (d. 1676) who was the son of John Sinclair, Master of Berriedale and his wife Jean, daughter of Colin Mackenzie, 1st Earl of Seaforth. The 6th Earl of Caithness's paternal grandparents were William Sinclair, Lord Berriedale and Mary, daughter of Henry Sinclair, Lord Sinclair. William Sinclair, Lord Berriedale was in turn the elder son of George Sinclair, 5th Earl of Caithness.
George Sinclair, 6th Earl of Caithness married Mary, daughter of Archibald Campbell, 1st Marquess of Argyll, but had no children. He died at Thurso Castle in 1676. The Earldom of Caithness being much in debt, he had desponed[check spelling] the estates and the title to his principal creditor, John Campbell of Glenorchy, who upon Sinclair's death was created Earl of Caithness by patent. This however was challenged by George Sinclair of Keiss. On 13 July 1680, Campbell of Glenorchy took a force of 800 men north to evict Sinclair of Keiss, who was waiting for him with 500 men near Wick. Inflamed with drink, the Sinclairs attacked the force of Campbells and were routed in what is known as the Battle of Altimarlach. Legend has it that so many Sinclairs were killed that the Campbells were able to cross the river without getting their feet wet. However, in 1681 the Privy Council of Scotland made a proclamation in favor of Sinclair of Keiss who became the 7th Earl of Caithness and Campbell of Glenorchy was created Earl of Breadalbane and Holland.
George Sinclair of Keiss, 7th Earl of Caithness died in 1698 without issue and this brought an end to the male heirs of the 5th Earl. The Earldom of Caithness then devolved upon the male heirs of James Sinclair, 1st of Murkle, who himself was a son of John Sinclair, Master of Caithness (d. 1576), who in turn was a son of George Sinclair, 4th Earl of Caithness (d. 1582). John Sinclair, eldest son of James Sinclair, 2nd of Murkle, then became the 8th Earl of Caithness.
- Henderson, John W.S (1884). Caithness Family History. Edinburgh: David Douglas. pp. 7-9.
- Anderson, William (1862), The Scottish nation: or, The surnames, families, literature, honours, and biographical history of the people of Scotland, 1, Fullarton, pp. 524–525
- Way, George of Plean; Squire, Romilly of Rubislaw (1994). Collins Scottish Clan & Family Encyclopedia. HarperCollins Publishers. pp. 322–323. ISBN 0004705475.
- Henderson, John W.S (1884). pp. 5-7.
|Peerage of Scotland|
| Earl of Caithness