From Destination Delco:
Delaware County is the fifth most populous county in Pennsylvania…
and the third most compact. The county was created on September 26, 1789, from part of Chester County, founded in 1682, and named for the Delaware River that defines its southeastern border.
Though small in size, Delaware County is large in history. The William Penn Landing Site Monument at Penn and Front Streets in Chester marks the spot of the first landing of Penn on the territory of Pennsylvania in 1682.