The William Young Brown Collection is comprised of approximately 3 feet of manuscripts. It consists primarily of sermons, but also contains lectures. The dates span from 1861 to 1913. The sermons were given primarily in Darby, PA, and often repeated in Narberth, PA, and Boone, IA, although some sermons are original to other locations.

The collection consists of papers and other materials covering Gillespie's life as a student, pastor, professor, and administrator. Much of the material includes correspondence and other communications between Gillespie and others. The bulk of the collection spans his presidency at Princeton Theological Seminary.