Delaware County was created from Chester County on September 26, 1789. The current county seat is in Media and before that, the county seat was in Chester.
The land was explored by Henry Hudson in 1609 and was settled after that by the Swedes, the Dutch and the English. The original inhabitants were the Lenni-Lenape tribe.
King Charles II of England made a land grant to William Penn in 1682, who named the area Pennsylvania. Penn divided this area into three counties - Philadelphia, Bucks and Chester. When Chester County was divided, the southeastern portion became Delaware County, taking its name from the Delaware River.