 Grant was born at Blairfindy, Scotland in 1744, the son of William Grant (d.1762), Laird of Blairfindy, and Jean Tyrie.In 1759, he was hired by a relative, Robert Grant, who was supplying the Royal Navy at Quebec and sent there as an agent of the company.While there, he became involved in other business on his own, including trade in the seal and salmon fisheries, grain and furs.