This short article reports on a previously undocumented manuscript of John Brown's influential 'Description of the Lake at Keswick'. The 'Description' is an epistolary prose sketch of Derwent Water, in the English Lake District, and has long been recognised as a foundational document in the development of tourism in the region. The date when Brown wrote the text is not known, but evidence suggests that it may have been written during the early 1750s. Copies of the 'Description' appeared in various periodicals during the 1760s, and a chapbook edition of the text was printed in Newcastle-upon-Tyne in 1767. Subsequent chapbook editions appeared thereafter. Prior to publication, the 'Description' circulated privately in both manuscript and print. Until recently, however, none of these private copies were known to exist. This has been an impediment to further research. The manuscript documented in this article is therefore of interest. In addition to providing an account of the manuscript, the article considers the light it sheds on the publication history of Brown's 'Description'.