He inherited the Rosehaugh estates near Avoch in Ross-shire through his paternal grandmother Agnes Mackenzie and assumed the additional surname of Mackenzie. He was returned to Parliament for Buteshire in 1747, a seat he held until 1754, and then represented Ross-shire from 1761 to 1780. In 1761 he was sworn of the Privy Council. In 1763 he became Keeper of the Privy Seal of Scotland until 1765 and then again in 1766 until his death.
In 1752 Hon. James Stewart Mackenzie sold Rosehaugh and bought from Sir Thomas and William Nairn the ecclesiastical lands of Kirkhill in Meigle, which belonged to the Holy Trinity of Dunkeld, and upon the site of the former Castle, which had been used as a grange for the Churchmen, he erected Belmont Castle, at a cost of £10,000.[2]
He was a very studious man and a great astronomer. A telescope, purportedly specially made for him, is in the Robert Whipple Collection at the University of Cambridge.[3] He was responsible for the building of the observatory on Kinpurnie Hill, then part of his estate. The observatory was designed by Alexander Bryce (1713 - 1786), Minister of Kirknewton and East Calder, but remained incomplete.
He died on 6 April 1800.
Family
Stuart-Mackenzie married his first cousin Lady Elizabeth Campbell,[4] daughter of John Campbell, 2nd Duke of Argyll, in 1749. They had no surviving children. She died in July 1799. Stuart-Mackenzie survived her by less than a year and died in April 1800. According to a decision in 1803 his estates were passed on to his nephew James Stuart-Wortley-Mackenzie.
^William Anderson, The Scottish Nation or, the Surnames, Families, Literature, Honours, and Biographical History of the People of Scotland. (Edinburgh: Fullarton, 1877), I: 517.
