Mackenzie spent much of his youth on the continent. James Boswell, who met him at the court of Brunswick in 1764, thought him ‘a lively pretty young man with the most perfect elegance of manners, having been abroad a great many years’.
Fortrose’s attendance was poor; and his only known votes were for the naval captains’ petition, 9 Feb.1773 (marked in the King’s list as a ‘friend’), and with Government over the Middlesex election motion, 26 Apr. 1773. There is no record of his having spoken in the House. On Grafton’s rejoining the Government in 1771 Fortrose was created Earl of Seaforth in the Irish peerage; and at the end of the Parliament was listed by Robinson as a Government supporter.
Caithness was not represented in the 1774 Parliament; but Robinson mentioned Fortrose as a possible candidate for Tain Burghs, for Inverness-shire, and also for Ross,
In 1777 Seaforth offered to raise a Highland regiment; but recruiting was slow, and the King made difficulties over Seaforth’s nominations of officers; finally the regiment was embodied in May 1778. In 1779 Seaforth, because of vast debts, sold his estates to his second cousin and heir male, Thomas Mackenzie Humberston, and with them his interest in Ross-shire. He sailed with his regiment for India in June 1781 and died at sea, 27 Aug. 1781.
