The main interest of the papers lies in the military and diplomatic material relating to General St Clair's career in the British army over thirty years. The 2nd son of the 10th Lord Sinclair, James St Clair succeeded as titular 12th Lord Sinclair in 1750 on the death of his brother, who was attainted in 1715. In 1747 he married Janet, daughter of Sir David Dalrymple, 1st Baronet of Hailes, and widow of Sir John Baird, 2nd Baronet of Newbyth. He served with the Foot Guards in Gibraltar in the 1720s and 1730s, and later under Cumberland in Flanders and England in 1754, was Commander-in-Chief of the proposed expedition against Canada in 1745, commanded the unsuccessful attack on the French coast at L'Orient in 1745, and served as special envoy to Vienna and Turin in 1748. General St Clair was Member of Parliament for Dysart Burghs from 1722 to 1734, Sutherland from 1736 to 1747, again for Dysart Burghs from 1747 to 1754, and Fifeshire from 1754 to 1762.