When it was founded in Boston in 1860, the General Theological Library declared that its mission would be to "maintain a library of all works pertaining to theology and religious knowledge and. . . to promote the interest of religion and the diffusion of theological learning." Over the last century and a half, the organization has followed that mission and has evolved into a focal point for religious learning in northern New England.

Initially, the Library supplied theological materials by mail to New England ministers who found themselves in remote rural areas after graduating from seminary. As the Library grew, so did the geographic dispersion of its members, and soon the institute was sending books all across the country.

In 1988, a dedicated and diverse group of Maine clergy and laypersons negotiated a deal to move the collection to its present location in Portland's historic State Street Church. The revitalized Library continued to loan books to its mail-order patrons while it increased its presence in southern Maine by offering religious workshops and lectures.

The move also coincided with the Bangor Theological Seminary's plan to expand into the Portland area. After a few years of supporting the Library's Portland operations, the seminary founded a new campus at the State Street Church in 1991. The seminary then assumed responsibility for the General Theological Library's collection and continued its mission of promoting theological learning.