In these epistles he assailed Mansfield for his want of impartiality with a force and eloquence that caused him at the time to be regarded as a worthy rival to Junius.
From 1777 to 1781 he was occupied with the affairs of his younger brother, Colonel James Stuart (died 1793), who had been suspended from his position by the East India Company for the arrest of Lord Pigot, the governor of the Madras presidency.
He published several letters to the directors of the East India Company and to the secretary at war, in which his brother's case was set forth with great clearness and vigour.
On 19 July 1790 he re-entered parliament, after an absence of six years, as member for Weymouth and Melcombe Regis, for which boroughs he sat until his death.
The youngest, Charlotte, in 1830 married Robert Harington, younger son of Sir John Edward Harington, eighth baronet of Ridlington in Rutland; through her, on the death of her elder sisters, the estate of Torrance descended to its present [1898] occupier, Colonel Robert Edward Harington-Stuart, while Castlemilk reverted to the family of Stirling-Stuart, descendants of William Stirling of Keir and Cawder, who married, in 1781, Jean, daughter of Sir John Stuart of Castlemilk.
Some notes made by him in July 1789 on charters in the Scottish College at Paris are preserved in the Stowe MSS.