In 1741 Mackay stood unsuccessfully for Sutherland against General James St. Clair, who was supported by his nephew, the Earl of Sutherland. In 1745, under the threat of rebellion, his father made proposals to the Earl of Sutherland for a bond of friendship. In formal documents signed at Tongue and Dunrobin in July 1745 they pledged themselves to bury all differences ‘in everlasting oblivion’, to unite in the King’s cause, and to act ‘in mutual concert’ in all future parliamentary elections.
During the rebellion Mackay, as captain of an independent company raised by his father, distinguished himself in March 1746 in the action near Tongue, resulting in the capture of recently landed French money, for which he obtained a regular commission
He died 25 June 1782.
