In one of his final letters, written in 1661, Baillie exhorted William Cunningham, eighth earl of Glencairn, to obey ‘Prince, Countrey, and Mother-Church’ after God, so that in the ‘true account I may readilie give to the world and posteritie of what is past among us these thirty-six years, your Lordship's just character may be with the fairest of all’. Baillie never began writing the history of the British Civil Wars promised in this letter, but he did bequeath to posterity a detailed account of contemporary events via transcribed copies of his outgoing letters. His decision to transcribe painstakingly a selection of outgoing letters and accompanying documents was motivated by his desire to compile a body of evidence that could be used to prepare an historical account of the Covenanters. In this vein, Baillie showed remarkable historical awareness that has benefitted generations of historians from the 1640s to the present day. He recognized the importance of the conflicts which he witnessed and he decided, quite early on, that he was well placed to document and narrate current events. In his own lifetime, his cousin and a small circle of historians based in northern Europe exploited his letters to these ends. But Baillie's efforts as manuscript collector were also a means by which he unintentionally fashioned his own legacy as detached chronicler, rather than concerned participant.

David Laing's edition of Baillie's Letters and Journals is still a valuable resource for scholars of mid-seventeenth-century Britain. Nevertheless, Baillie's practice as contemporary historian, life-writer and manuscript collector remains unstudied. Often, citation of a letter written by Baillie carries with it the tacit assumption that the text survives simply to provide historians with details about an event that fell between 1637 and 1662. Baillie has been variously taken as representative of a ‘Scottish’ or a ‘Covenanting’ opinion on a debate in the Westminster Assembly; he has been cited as an ‘eyewitness’ to treaty negotiations in 1640, 1646 or 1649; and he has been quoted for representative flavour of ‘popular’ reaction to Laudian liturgical reforms.