Delaware County, Pennsylvania, is the oldest settled section in the state, having been first settled in 1643 by the Swedish Governor Johan Printz. Equitable treatment of the nearby Lenni Lenape Indians helped these people survive, and later, thrive.
After a brief period of time, the settlements in the area fell under control of England's Duke of York. In 1681, King Charles II gave proprietary colony rights to William Penn, which lead to the founding of The Province of Pennsylvania under Penn's control.
A year later, in 1682, the General Assembly of Pennsylvania passed its "Great Law of Pennsylvania" that gave citizens, among other things, freedom of religion and the right to vote. During this period of time, the population of the county tripled and many roads were built, some of which are still used today.
During the American Revolution, the county saw numerous battles, the largest of which was the Battle of Brandywine River, later to be known as the largest land battle of the Revolution. It took place on Sept. 11th, 1777.
Some years later, in 1789, Delaware County split from Chester County with the county seat still remaining in Chester. Another county seat, Media, was later moved to the interior in 1850, which caused many land owners to sell their land and move near there. This allowed for explosive industrial development as industries bought up the available land. The resulting boon laid the foundation for exponential population growth and residential development, leading to the county becoming the 4th most populated in the United States.