Sinclair’s father succeeded his own father as the 7th baronet of Mey in 1774, at the age of eight. In 1789 he succeeded his cousin John Sinclair, a suicide, as 12th earl of Caithness, but he did not assume the title until 4 May 1793, when his right was confirmed by the Lords. He was appointed lord lieutenant of Caithness, where his substantial estates centred on Barrogill Castle, near John O’Groats, in 1794, and postmaster-general of Scotland by the Perceval ministry in 1811. He was a Scottish representative peer, 1807-18. With his wife, a niece of Sir John Sinclair of Ulbster, Member for Caithness, 1790-6, 1802-6, 1807-11, he had six sons, of whom the eldest, John, died young in 1802, leaving the second, Alexander Campbell, heir to the earldom as Lord Berriedale. His next brother James Sinclair followed him into the army in 1814, but his career was desultory. When the 12th earl died impoverished in 1823 his successor applied successfully to the Liverpool ministry for the lord lieutenancy of Caithness, but failed to secure their support for his pretensions to a vacancy in the representative peerage, which had already been promised to Lord Errol. His wife Frances complained directly to her cousin Canning, the foreign secretary, and insinuated that Lord Caithness might turn against government, but she received a flea in her ear.
He made no mark in the House, from which he was given six weeks’ leave to attend to urgent business, 11 Apr. 1827. He voted with the duke of Wellington’s ministry against repeal of the Test Acts, 26 Feb., and paired for Catholic relief, 12 May 1828. In January 1829, complaining that he and his wife were virtually destitute as a result of the bankruptcy of his father-in-law and that he had spent heavily to secure his election, he solicited a small pension for his wife, but to no avail.
