Adams County was originally included within the ample limits of Chester County. Soon after the settlement of Pennsylvania by William Penn, 1682, the Province was divided by its proprietor, into three counties, Bucks, Chester and Philadelphia. Lancaster County was separated from Chester by Act of May 10, 1729, and was the first county established subsequent to the formation of the three original counties. The first division of Lancaster County was by act of August 9, 1749, when York County was separated from it. York, which then included what is now Adams, was the first county erected west of the Susquehanna River, and embraced all that territory bounded on the west and north by the South Mountain, on the east by the Susquehanna, and on the south by Maryland. The county being very large, and the distance from the upper end to the county seat being great, a movement looking to the formation of a new county was set on foot as early as 1790. Much feeling was soon developed in reference to this matter. Those living within easy reach of the old county town manifested their selfishness by violently opposing the measure, while those residing within the limits of the proposed new county were just as active and zealous in favor of the separation. Public meetings were held, petitions for and remonstrances against the erection of a new county were industriously circulated, signers to each obtained, and presented to the Legislature. Finally after ten years of contention and strife, the separation took place by virtue of an act of Assembly dated January 22, 1800. The new county was named Adams, in honor of John Adams, who as President of the United States from 1797 to 1801. The commissioners to mark and run the line dividing Adams from York county were Jacob Spangler, deputy surveyor of York county, Samuel Sloan, deputy surveyor of Adams county, and William Waugh.
Information taken from History of Pennsylvania by William H. Egle, M.D., M.A.