William St. Clair (1404-1482), third Earl of Orkney, first Earl of Caithness

He was the third and last Sinclair Earl of Orkney and he was the grandson of Prince Henry Sinclair, first Sinclair Earl of Orkney. In 1470 King James III of Scotland claimed the Earldom of Orkney for the Scottish crown, conveying it to his daughter Margaret, who at the age of eleven was being married to the Dauphin of France; the rents of Orkney were her dowry.

In 1455 Earl William, while still the Norse Earl of Orkney, was granted the Earldom of Caithness by King James II of Scotland by letters patent. He thus became the first Sinclair Earl of Caithness and the first Earl of Caithness under Scottish law. However, he was the thirty ninth Earl of Caithness since the first Norse Jarl of Orkney and Caithness. He transferred the title to his son in 1476 before his death.