He was the son of William Sinclair, 2nd Earl of Caithness and Margaret, daughter of Sir Gilbert Keith of Inverugie.

He received a charter from the Crown for himself and his wife Elizabeth Sutherland of Duffus that was dated 14 July 1527 for the lands of Keiss, Stane, and Rowdale in the earldom of Caithness and sheriffdom of Inverness.

This charter also granted to William Sinclair, the earl's son and heir apparent, the earldom of Caithness under reservation of his father's life-rent, and his spouse's reasonable terce.

Gavin Dunbar, Bishop of Aberdeen pronounced his decision at Edinburgh on 24 March 1524, and both parties were satisfied and the two Earls lived in peace with each other from then onward.

The Earl of Caithness coming to the aid of his kinsmen was slain with many of his followers at the Battle of Summerdale on 18 May 1529.