George Sinclair, 5th Earl of Caithness was the eldest son of John Sinclair, Master of Caithness and his wife Jean, daughter of Patrick Hepburn, 3rd Earl of Bothwell. His father, the Master of Caithness, had obtained a charter from Mary, Queen of Scots by which the Earldom of Caithness became a male fee, to him and his male heirs. George Sinclair, 5th Earl of Caithness therefore succeeded his grandfather, George Sinclair, 4th Earl of Caithness in 1583.
He was Sheriff of Caithness. This led to a dispute with George Keith, 5th Earl Marischal in 1610 because his tenants in Caithness were not subject to the Sheriff's jurisdiction. After a law suit from George Sinclair of Dunbeath, Marischal sold his Caithness lands to the Earl.
- William Sinclair, Lord Berriedale, who married Mary, daughter of Henry Sinclair, Lord Sinclair. William died before his father, leaving a son, John Sinclair, Master of Berriedale, who married Jean Mackenzie, daughter of Colin Mackenzie, 1st Earl of Seaforth. John Sinclair died in 1639, leaving three sons: George Sinclair, 6th Earl of Caithness, John Sinclair and William Sinclair.
- Francis Sinclair of Northfield, Edinburgh, who married Elizabeth, daughter of Lord Fraser, and had a son, George Sinclair, 7th Earl of Caithness (originally of Keiss). Francis also left a daughter, "Lady Jean of Mey", who died in 1716. Francis also had a natural (illegitimate) daughter, Margaret, who in 1653 married John, son of Alexander Sutherland in Lybster.
- Francis Sinclair, a natural (illegitimate) son who in about 1621 fought a duel with his relative, William Sinclair of Mey. From this Francis Sinclair descends the Sinclair of Stirkoke branch which includes George Sinclair who was killed in an expedition to Norway in 1612.
- John Sinclair, a natural (illegitimate) son who attained the rank of Lieutenant-Colonel in the German wars.
|Peerage of Scotland|
| Earl of Caithness