From 1771 to 1792, much of the correspondence is of the Honourable Mary Cathcart, afterwards Mrs Graham. It has been used extensively in 'The beautiful Mrs Graham and the Cathcart Circle'.

Throughout, the letters deal with family affairs, the business of the Balgowan estate and other business, social life in Great Britain and abroad, and hunting. The military operations in which Sir Thomas Graham was engaged are illustrated by correspondence on the campaign in northern Italy, 1796-1799, the reduction of Minorca, 1798-1799, his Governorship of Messina, 1799, the conquest of Malta (with the formation of the Maltese Light Infantry), 1800, the Walcheren expedition, 1809-1810, the Peninsular War (especially Cadiz and St Sebastian), 1808-1813, and the expedition against Antwerp, 1813-1814. The letters of 1799-1800 include a large number from naval officers on operations in the Mediterranean. Other letters deal with the 90th Foot, raised by Graham, 1794-1825, undated, the 1st Foot, commanded by him, 1834-1839, and the foundation of the United Service Club, 1815-1816.