He was the son of John Sinclair, 3rd Earl of Caithness and Elizabeth, daughter of William Sutherland of Duffus. He was the Justiciar of Caithness by a grant from Mary, Queen of Scots in 1566. He was also one of the peers who sat on the trial of Lord Bothwell. In 1570, the Battle of Torran-Roy took place between the forces of George Sinclair, 4th Earl of Caithness and Alexander Gordon, 12th Earl of Sutherland. Caithness was initially defeated by Sutherland's vassals the Murrays of Aberscross, but he returned to besiege the Murrays at Dornoch after which several of them were beheaded. The Earl of Caithness later imprisoned his son, the Master of Caithnes, for making peace with the Murrays. The Master of Caithness died at Castle Sinclair Girnigoe in 1576.
- John Sinclair, Master of Caithness (d.1576), who married Jean, daughter of Patrick Hepburn, 3rd Earl of Bothwell, and left children, including George Sinclair, 5th Earl of Caithness.
- William Sinclair, first laird of Mey and ancestor of the Sinclairs of Ulbster. His son William was at High School in Edinburgh in 1595 and shot John MacMorran.
- George Sinclair of Mey, Chancellor of Caithness.
- Barbara Sincalair, who married Alexander Gordon, 12th Earl of Sutherland and divorced him by 1573.
- Elizabeth Sinclair, who married firstly, Alexander Sutherland of Duffus and secondly, Huistean Du Mackay, 13th of Strathnaver.
- Another daughter married to Alexander Innes of Innes.
- Janet Sinclair, second or third wife of Robert Munro, 16th Baron of Foulis, but who died childless. In 1582 she received a Tack of the Parsonage of Spittal.
- Henderson, John W.S (1884). Caithness Family History. Edinburgh: David Douglas. pp. 5-6.
- Maclauchlan, Thomas, 1816-1886; Wilson, John, 1785-1854; Keltie, Sir John Scott, 1840-1927 (1875). A History of the Highlands and of the Highland Clans. 1. Edinburgh: T.C. Jack. pp. 92-93.
- Way, George of Plean; Squire, Romilly of Rubislaw (1994). Collins Scottish Clan & Family Encyclopedia. HarperCollins Publishers. pp. 322–323. ISBN 0004705475.
- Mackenzie, Alexander (1898). "XVI Robert Munro". History of the Munros of Fowlis. Inverness: A. & W, Mackenzie. p. 61.
|Peerage of Scotland|
| Earl of Caithness