William Sinclair(c. 1408-c.1484), 3rd Earl of Orkney, was born about 1410 and died about 1484. His father was Henry Sinclair(1375-1422), 2nd Earl of Orkney. He was also Baron of Roslin in Scotland.
William was the founder of Rosslyn Chapel in Midlothian, Scotland. After he took control of his inheritance of Nithsdale, he exchanged it for the estates of the earldom of Caithness, which had been held for generations by his family. Shortly thereafter he was given the title of Earl in Scotland. He also held the Norwegian Earldom of Orkney until 1470. At that time, King James III gained the rights to this earldom for the Scottish Crown. This exchange of ownership from Norway to Scotland came about because Christian I of Norway's daughter married James III of Scotland and it was given as her dowry. Since William Sinclair had held this land under the king of Norway, James III gave him Ravenscraig in return for his giving up the Earldom of Orkney. An act of parliament, February 20, 1472, annexed the Earldom of Orkney to the Scottish crown.
He was the grandson of Henry Sinclair, 1st Earl of Orkney and son of Henry Sinclair, 2nd Earl of Orkney, for a time 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 1449.
The first important territorial transaction that William made during his lifetime, was the exchange of his inherited lordship of Nithsdale to the estates of the earldom of Caithness - which soon led to his obtaining the title of Earl in the peerage of Scotland.
William Sinclair had three wives during his lifetime. He married Lady Elizabeth Doughlas first. She was the daughter of Archibald Douglas, 4th Earl of Douglas. His second wife was Marjory Sutherland. She was the daughter of Alexander Sutherland. His third wife was Janet Yemean.
He had two children by Elizabeth Douglas. William Sinclair, 2nd Lord St. Clair, was their son. He was disinherited because his father considered him to be a wastrel. The only inheritance he received was the Castle of Ravenscraig in Fife. They may have also had a daughter named Marjory or Elizabeth Sinclair. She married Andrew Leslie, Master of Rothes. Most sources say that she was the daughter of his second wife Marjory Sutherland. Cokayne's Peerage says her name was Marjory.
William Sinclair had four children by his second wife Marjory Sutherland: Eleanor Sinclair, Catherine Sinclair, Oliver Sinclair, and another son named William Sinclair, who became the 2nd Earl of Caithness. The other son of this marriage, Oliver Sincliar, received the Barony of Roslin as his inheritance.
William and Marjory's daughter, Lady Eleanor Sinclair married John Stewart, 1st Earl of Atholl. James VI of Scotland descends from Eleanor. Their other daughter, Catherine Sinclair, married Alexander Stewart, 1st Duke of Albany. He was the nephew of her sister's husband, the Earl of Atholl.
In Walter Scott's Lay of the Last Minstrel, there is a song called the ballad of Rosabelle. It is sung by the bard of the Lord St. Clair, and is supposedly about the lord's daughter. In the ballad, she drowned, after setting out on a boat from Ravencraig, crossing the Firth of Forth to the family's other castle of Roslin, in Midlothian.
Rosslyn is sometimes spelled Roslin. Rosslyn Chapel and Castle are near the village of the same name in Midlothian. It lies about 9 miles from Edinburgh. The castle was built by William's
Much of this information can be found the Burke's Peerage and Baronetage, 107th edition, by Charles Mosley.