The Library has collected manuscripts and archives since the 1650s when it built up an impressive collection of medieval manuscripts. By the end of the eighteenth century, however, Chetham's was increasingly seen as a repository for local records, indeed the only proper repository for public papers in the region. In the nineteenth century, as the Library narrowed its subject coverage from the whole range of knowledge to history and topography, the collection and study of documents concerning the history of the north west took on greater significance. Many of the local manuscripts and archival holdings came to Chetham's in the second half of the nineteenth century, often through members of local historical societies, particularly the Chetham Society.
The Library holds over 1,000 volumes of manuscripts and tens of thousands of original and secondary documents. The overwhelming majority of material is of local interest, and is concerned primarily with the area now represented by the County of Greater Manchester. Particular strengths are Manchester, Salford and the area north of Manchester - Rochdale and neighbouring towns. To a large extent, the Library's holdings are less strong for areas outside the immediate region: for the north and west of Lancashire and much of Cheshire the Library is relatively weak in comparison with other libraries and record offices.