Franklin County was erected September 9, 1784, and its territory taken from Cumberland. The following day, September 10, 1784, Montgomery County was formed from a part of Philadelphia County, the last territory to be taken from the original county.
March 4, 1785, Dauphin was cut off from Lancaster; September 25, 1786, Luzerne was erected from Northumberland, and September 20, 1787, Huntingdon was formed from Bedford.
Allegheny County was formed from Westmoreland and Washington Counties, September 24, 1788. Mifflin was formed from Cumberland and Northumberland Counties September 19, 1789.
Old Chester County lost part of its territory when Delaware County was cut from it September 26, 1789. Thus the county which comprised the most ancient settlements in Pennsylvania was now formed into the new County of Delaware, and the organization of counties in the southeastern part of the State completed.
When the County of Lycoming was cut from Northumberland, April 13, 1795, it was for years the largest county of the State. Four days later the County of Somerset was formed from Bedford. Green County was cut from Washington February 9, 1796, thus completing the formation of counties in the southwest corner of the State.
The next county to be formed was that of Wayne, which was set off from Northampton March 21, 1797, and formed the northeastern corner of the State.
Adams was erected from York January 22, 1800, and February 13 following Center was formed from parts of Northumberland, Lycoming, Mifflin and Huntingdon, and March 12 eight new counties—Armstrong, Beaver, Butler, Crawford, Erie, Mercer, Venango and Warren—were formed. Thus, the remaining corner of the State was organized. The counties were taken from Lycoming and Allegheny, Westmoreland furnished a part of Armstrong and Washington yielded up a portion for Beaver, but Allegheny furnished the largest amount of territory for the new counties.
Indiana was cut from Westmoreland and Lycoming, March 30, 1803.
Six new counties were erected on March 26, 1804, when Cambria, Clearfield, Jefferson, McKean, Potter and Tioga were formed. The latter four being taken from Lycoming, while Northumberland helped with Clearfield, but Cambria was cut from parts of Huntingdon, Somerset and Bedford. Bradford and Susquehanna, were formed February 21, 1810, the former from Lycoming and Luzerne and the latter from Luzerne alone.
March 11, 1811, Schuylkill was formed from Berks and Northampton. March 6, 1812, Lehigh was taken from Northampton, and February 16, 1813, Lebanon was erected from Lancaster and Dauphin. Columbia and Union were erected March 22, 1813, both being taken from Northumberland. Pike was cut from Wayne, March 26, 1814, and Perry was taken from Cumberland, March 22, 1820.