Duncan Forbes was born on 10 November 1685, the second son of Duncan Forbes of Culloden and Bunchrew, near Inverness. He went to Edinburgh University to study law, but in 1705 proceeded to Leiden instead to study civil law and oriental languages, returning to Scotland in 1707. He was admitted as an advocate in 1709, and was also appointed Sheriff of Midlothian around the same time. During the 1715 Jacobite Rebellion, his brother John Forbes (who had joined with Simon Fraser, 12th Lord Lovat) accompanied him to Inverness where they raised forces in support of the government. Duncan was rewarded with the office of Depute-Advocate which he took up in 1716.

In 1721-22, Forbes was MP for Ayr, and in 1722 he was returned in Inverness. In 1725 he was appointed Lord Advocate in succession to Robert Dundas of Arniston.

As the English system of taxation was extended into Scotland troubles began to arise and rioting took place in Glasgow in 1725. Forbes accompanied a force commanded by General Wade, and the Glasgow magistrates were arrested for their negligence and brought to Edinburgh with some of the rioters. They were later freed. Edinburgh too suffered trouble within the brewing industry at this time.

In 1735, Forbes succeeded to the family estates on the death of his brother. In 1737 he opposed the Bill bringing penalties on the City of Edinburgh after the Porteous Riots. In the wake of the 1745 Jacobite Rebellion, he protested against the cruelties of the Duke of Cumberland.

Duncan Forbes died on 10 December 1747.