Banker and philanthropist. Born in Edinburgh, the son of a lawyer who died when Forbes was only four years of age. He and his mother were sustained by his uncle, Lord Pitsligo.
He entered the bank of Coutts & Co. in Edinburgh as an apprentice in 1754, rising to become a partner. When one of the Coutts died, another retired and the remaining two settled in London, the bank changed its name to Forbes, Hunter & Co. (1763), reflecting a new partnership which included Sir James Hunter Blair. It became the Union Bank in 1830.
Forbes supported many charitable works on Edinburgh, including an orphan's hospital, blind asylum and a work-house. He was also central to the building of a new High School in 1777. He acquired the lands of the Barony of Pitsligo in 1781 and greatly improved the area, laying out the village of New Pitsligo in 1783 and giving generously to the local people.
Forbes was a founder member of the Society of Antiquaries of Scotland (1780) and the Royal Society of Edinburgh (1783). His grandson was Professor of Natural Philosophy James David Forbes (1809-68).