He married Lady Margaret Primrose, daughter of the Earl of Rosebery.
He died in 1765, leaving an only child, Lady Dorothea, who married James Duff, 2nd Earl Fife and who died in 1819 without issue.
He had resided at Haimer Castle, which after his death fell into disrepair.
However, his grandson, James, who did claim the title, failed to establish legitimacy of his father, David, the son of David Sinclair of Broynach.
William Sinclair of Rattar was therefore served male heir, becoming 10th Earl of Caithness, with the Committee of Privileges adjudging the title to him in May 1772.