The great majority of the letters are to Lord Milton; there are some to other members of his family (especially Lady Milton and his son Andrew) and some to others which for various reasons came into his possession (including intercepted rebel letters during the '45); there are also a number of scrolls and copies of his replies.

The correspondence reflects all Lord Milton's many activities, especially in his public offices and his capacity as friend and agent of the 3rd Duke of Argyll. Detailed accounts of political events in London were supplied to him (by at different times Colonel John Middleton, John Maule, Lord Somerville, and Gilbert Elliot; see also family correspondence). Scottish affairs however predominate: outstanding events such as the Porteous Riots and the Jacobite Rebellion, but also less violent matters such as Parliamentary elections, local politics especially in Edinburgh, the administration of justice, the affairs of the Church of Scotland and the Universities of Glasgow and Aberdeen, the Customs, and patronage at all levels. In financial and industrial matters he was closely concerned with the Royal Bank of Scotland and the British Linen Company, as well as the encouragement of manufactures in general. In addition to being a landowner himself (at Brunstane and Saltoun) he was in general charge of the Argyll estates. Other topics which recur frequently are: the affairs of his numerous relations and friends, legal questions, postal services, and the promotion of the education and careers of his children.