The following describes the territory from which Mercer County was originally formed.
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| Chester County was one of the original counties of the province. |
| May 10, 1729 : Lancaster County formed from part of Chester County. |
January 27, 1750: Cumberland County formed from part of Lancaster County.
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| March 9, 1771: Bedford County formed from part of Cumberland County. |
| February 26, 1773: Westmoreland County formed from part of Bedford County. |
| October 1784: The region covering
about twenty counties in the northwestern part of Pennsylvania was
purchased from the Six Nations by the Commonwealth, at the treaty of
Fort Stanwix (now Rome, N.Y.) . |
| 1785: A part of the purchase of 1784 was added. |
| September 24, 1788: Allegheny County formed from parts of Westmoreland and Washington Counties. |
| March 12, 1800: Beaver and Mercer Counties formed from part of Allegheny County. |
| April 5, 1849: Lawrence County created from parts of Beaver and Mercer Counties. |