He was the grandson of the explorer Henry Sinclair, 1st Earl of Orkney and son of Henry Sinclair, 2nd Earl of Orkney, for a time protector of the young James Stuart, the later James I of Scotland. He was Lord High Admiral of Scotland, and was Lord Chancellor of Scotland from 1454 to 1456. He was First Lord St. Clair in Scotland 1449.
King James III gained the Earldom of Orkney for the Scottish Crown in 1470 (see History of Orkney), and William Sinclair was thereafter Earl of Caithness alone until he resigned the Earldom in favour of his son William in 1476.
In 1471 James bestowed the castle and lands of Ravenscraig in Fife on William Sinclair, in exchange for all his rights to the earldom of Orkney, which, by an Act of the Parliament of Scotland, passed on February 20, 1472, was annexed to the Scottish crown.
He was married three times, first to Lady Elizabeth Douglas, daughter of Archibald, 4th Earl of Douglas; secondly to Marjory Sutherland, daughter of Alexander Sutherland, and lastly to Janet Yeman.
He fathered two known children with Lady Elizabeth Douglas. Their son (William Sinclair, 2nd Lord St. Clair) was, in the opinion of the father, a wastrel, whereby he was disinherited consequently. His family received only the Castle of Ravenscraig. Their daughter (Elizabeth Sinclair) would marry Andrew Leslie, Master of Rothes.
He fathered four known children with Marjory Sutherland; Eleanor Sinclair, Catherine Sinclair, Sir Oliver Sinclair, and William Sinclair, 2nd Earl of Caithness.
The earl's third son (William Sinclair, 2nd Earl of Caithness), of his second marriage became the designated heir of the Earldom of Caithness, and continued that title. The Barony of Roslin went to his second son (Sir Oliver Sinclair).
All in all, the Sinclair ancestry is well represented in Scottish and British high nobility, thanks to marriages of his daughters and other descendants.
William's daughter of his second marriage, Lady Eleanor Sinclair, married John Stewart, 1st Earl of Atholl, a relative of the kings. Lord Henry Darnley and his son James I of England descend from Eleanor, and through them, quite a many royal house of Europe. His other daughter by this marriage, Catherine Sinclair, married Alexander Stewart, 1st Duke of Albany.
Charles Mosley, Burke's Peerage and Baronetage, 107th edition.