William Sinclair (1410–1484), 1st Earl of Caithness (1455–1476), last Earl (Jarl) of Orkney (1434–1470 [de facto], -1472 [de Jure]), Baron of Roslin, was a Norwegian and Scottish nobleman and the builder of Rosslyn Chapel, in Midlothian.
His father Henry, who had been a de facto Jarl 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.
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.
William Sinclair was married three times: firstly to Lady Elizabeth Douglas, daughter of Archibald Douglas, 4th Earl of Douglas; secondly to Marjory Sutherland (married 1456), 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 Catherine Sinclair, who married Alexander Stewart, Duke of Albany.
- Elizabeth Sinclair, who married Andrew, Master of Rothes
By Marjory Sutherland he had the following children:
- Sir Oliver Sinclair, who received the Barony of Roslin.
- William Sinclair, 2nd Earl of Caithness (b. 1460 - d. 1513)
- Alexander Sinclair (c. 1454)
- George Sinclair (c. 1453)
- Robert Sinclair (1447)
- Arthur Sinclair (c. 1452)
- Lady Eleanor Sinclair (b. 1457- d. 1518), who married John Stewart, 1st Earl of Atholl.
- Lady Elizabeth Sinclair (b. c. 1455 - d. 1498), who married the Laird of Houston.
- Lady Margaret Sinclair (c. 1450), who married David Boswell of Balmuto.
- Lady Katherine Sinclair (b. 1440 - d. 1479)
- Lady Susan Sinclair (c. 1451)
- Lady Marjory Sinclair (1455–80), married Andrew Leslie, Master of Rothes. With Andrew she had issue including William Leslie, 3rd Earl of Rothes
- Lady Mariota Sinclair (c. 1455)
- Lady Euphemia Sinclair (c. 1470), who married John Kincaid Laird of Warriston.
- 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.
- Thomson, William P.L (2008). The New History of Orkney. Edinburgh. pp. 174–179.
- Cowan, Ian B; Easson, David E (1976). Medieval Religious Houses. p. 225. ISBN 0582120691.
- Hamre, Lars (1968). Norsk historie frå omlag år 1400. Oslo. p. 128.
- "Ravenscraig Castle". undiscoveredscotland.co.uk. Retrieved 1 December 2019.}
- Paul, James. The Scots Peerage. vol ii. pp. 333–336.
- Henderson, John W.S (1884). Caithness Family History. Edinburgh: David Douglas. pp. 1-4.
- Thomson, William P.L. (1987). History of Orkney. Edinburgh. pp. 134–136.
- Charles Mosley, Burke's Peerage and Baronetage, 107th edition.
- Sir James Balfour Paul, The Scots Peerage : founded on Wood's ed. of Sir Robert Douglas's Peerage of Scotland; containing an historical and genealogical account of the nobility of that kingdom. Edinburgh 1904. 
|Peerage of Scotland|
| Jarl of Orkney
– 1472 (seizure)
Territory seized by Scotland
| Earl of Caithness
George Crichton, 1st Earl of Caithness
|Lord High Admiral of Scotland||Succeeded by|
David Lindsay, 5th Earl of Crawford
William Crichton, 1st Lord Crichton
| Lord Chancellor of Scotland
Andrew Stewart, 1st Lord Avandale