The ancestry of the Thayer Memorial Library dates back to 1790, when a committee of five men was chosen to frame the bylaws of the Lancaster Library (renamed the Lancaster Social Library after its first decade), the earliest predecessor of Lancaster’s free public library. The original membership of the Lancaster Social Library consisted of seventy-four proprietors. New proprietors could be added through elections by a two-thirds vote, payment of at least three dollars, and signing the articles of agreement. The bylaws of the Lancaster Library as framed in 1790 stipulated that the collection was to be housed not more than a mile-and-a-half from the meeting house.
During the sixty years of its existence, the Lancaster Social Library survived reorganization, the rewriting of its regulations and bylaws, the tardiness of some proprietors in paying their dues, and the vicissitudes of taste in reading. In 1850, the separation of Clinton from Lancaster raised the question of how to divide the collection for use by members now living in two towns. Consequently, the books were sold at auction, many of them to individuals who had been proprietors of the Social Library.