He was the grandson of Henry Sinclair, 1st Earl of Orkney, and the son of Henry Sinclair, 2nd Earl of Orkney and Egidia Douglas. His father Henry, who had been a de factoJarl of Orkney, died in 1420; William travelled to Copenhagen in 1422 to establish his claim to the Jarldom, but David Menzies was appointed instead, to rule as William's guardian until he came of age. In 1424, William succeeded in wresting de facto control of the earldom from his guardian, but it was not until 1434 that he was acknowledged as Jarl of Orkney by King Eric. In 1446 he laid the foundation stone of the Collegiate Church of St Matthew, commonly referred to as Rosslyn Chapel. After the death of childless King Christopher of Norway in 1448, Earl William was mentioned as a possible candidate for the vacant throne, as the jarl of Orkney was the highest ranking nobleman in Norway. However, there are no indications that he pursued this claim. For a time William was protector of the young James Stewart, the later James I of Scotland. He was Lord High Admiral of Scotland, and was Lord Chancellor of Scotland from 1454 to 1456. He became the first Lord St. Clair in Scotland in 1449. In 1468, King James III of Scotland married Margaret, daughter of King Christian I of Norway. Christian was unable to immediately provide a dowry. Instead, he promised that dowry would be provided at a later date, pledging the territory of the Jarldom of Orkney as security for his promise. In 1470, James III offered William the castle and lands of Ravenscraig in Fife, in return for William quitclaiming his rights in Orkney and Shetland, an offer William accepted. The Norse jarldom technically remained in existence, but William now only had authority over the mainland parts - Caithness and Sutherland. In 1472, it having become clear that the dowry was unlikely to be paid, James declared the Jarldom's territory to be forfeit to the Scottish Crown, to which it was annexed by an Act of the Scottish Parliament, on 20 February. William now wielded his authority under the king of Scotland, rather than of Norway. Exchanging his inherited lordship of Nithsdale for lands in Caithness, William was granted the hereditary title Earl of Caithness. He resigned the Earldom in favour of his son William in 1476, and lived another 8 years.
Family
William Sinclair was married three times: firstly to Lady Elizabeth Douglas, daughter of Archibald Douglas, 4th Earl of Douglas; secondly to Marjory Sutherland, daughter of Alexander Sutherland of Dunbeath; and thirdly to Janet Yeman. By Lady Elizabeth Douglas he had the following children:
William Sinclair, 2nd Lord St. Clair was, in the opinion of the father, a wastrel, wherefore he was disinherited. His family received only Ravenscraig Castle in Fife.
Lady Euphemia Sinclair, who married John Kincaid Laird of Warriston.
Illegitimate:
Sir David Sinclair of Sumburgh, died 1507.
The earl's second son of his second marriage, William Sinclair, became the designated heir of the Earldom of Caithness, and continued that title. The Barony of Roslin went to his first son by that marriage, Sir Oliver Sinclair. All in all, the Sinclair ancestry is well and thoroughly represented in Scottish and British high nobility, thanks to the 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 of Scotland. Lord Henry Darnley and his son, King James VI of Scotland, descended from Eleanor, and through them, many Royal houses of Europe. His other daughter by this marriage, Katherine Sinclair, married Alexander Stewart, 1st Duke of Albany, a nephew of the 1st Earl of Atholl.
