James Stuart-Mackenzie (30 October 1718 – 8 April 1800) was a Scottish politician and joint founder of the Royal Society of Edinburgh in 1783.
He was educated at Eton College 1728 to 1732 then travelled to Europe to study at the University of Leyden where he graduated in 1737.
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.
A telescope, purportedly specially made for him, is in the Robert Whipple Collection at the University of Cambridge.
The observatory was designed by Alexander Bryce (1713 - 1786), Minister of Kirknewton and East Calder, but remained incomplete.