John Sinclair, 8th Earl of Caithness was the son of James Sinclair 2nd of Murkle, who in turn was a grandson of John Sinclair, Master of Caithness (d. 1576), who in turn was a son of George Sinclair, 4th Earl of Caithness (d. 1582). He had succeeded to the Earldom of Caithness upon the death of his relation George Sinclair, 7th Earl of Caithness, in 1698, who had died without issue and this brought an end to the male heirs of George Sinclair, 5th Earl of Caithness. His father had resigned the lands of Murkle in favor of himself and of John in March 1644.
John Sinclair, 8th Earl of Caithness died in 1705, leaving the following children:
- Alexander Sinclair, 9th Earl of Caithness, heir and successor.
- John Sinclair, Lord Murkle, who was a Senator of the College of Justice, and who left no issue.
- Francis Sinclair of Milton of Lieurary, who left no issue.
- Lady Janet Sinclair, who married David Sinclair of Southdun in 1714 and had several children.
|Peerage of Scotland|
| Earl of Caithness