[a] Delaware County borders Philadelphia, the nation's sixth-most populous city, to its northeast.

[4] Delaware County lies in the river and bay drainage area named "Delaware" in honor of Thomas West, 3rd Baron De La Warr, Governor of the nearby English colony of Virginia.

The land was explored by Henry Hudson in 1609, and over the next several decades it was variously claimed and settled by the Swedes, the Dutch, and the English.

Once the Dutch were defeated and the extent of New York was determined, King Charles II of England made his grant to William Penn to found the colony which came to be named Pennsylvania.

Crum Creek was dammed in 1931 near Pennsylvania Route 252 to fill Springton Lake (also known as Geist Reservoir), an approximately 391-acre (1.58 km2)[7] drinking water reservoir maintained by Aqua America, the county's largest lake.

Under Pennsylvania law, there are four types of incorporated municipalities: cities, boroughs, townships, and exactly one town.

There are 49 municipalities in Delaware County: Census-designated places are geographical areas designated by the U.S. Census Bureau for the purposes of compiling demographic data.

[17] Delaware County voted for the Republican candidate all but once from 1860 through 1988, with the exception being Lyndon Johnson's national landslide of 1964.

In the 2004 election Democratic presidential candidate John Kerry won the county by 14 points.

Driving the county's Democratic shift have been longstanding trends in voter registration advantage and demographics.

Since the 2000 Census, the White population of the county has decreased from 80.3% to 68.5% as of the 2020 Census, while, the Black population has risen from 14.5% to 22.7%, driven by the gentrification of Philadelphia and University City neighborhood and rapid demographic shift in Upper Darby.

[23] After the election of Donald Trump in 2016, the county rapidly shifted blue as a result of increased Democratic turnout and less enthusiasm from often less conservative suburban Republicans.

In the 2019 elections for the Delaware County Council, Democrats swept the board and elected Monica Taylor, Elaine P. Schaefer, and Christine Reuther, gaining control of the county Council for the first time since the Civil War.

[25] As of 2020, all of Delaware County is located in the state's 5th congressional district, represented by Democrat Mary Gay Scanlon.

The district had been held for 20 years by Republican Curt Weldon until he was ousted by Joe Sestak, a retired admiral, in the 2006 U.S. House of Representatives election.

Also in the 2006 election, Democrat Bryan Lentz unseated Republican incumbent State Representative Tom Gannon in the 161st House district.

In 2010 Sestak ran for the senate seat vacated by Arlen Specter and was replaced by Republican Pat Meehan, who defeated Lentz, the Democratic candidate.

This unit of officers includes the position of controller, District Attorney, treasurer, sheriff, register of wills, recorder of deeds, prothonotaries, clerks of the court, and the coroner.

[31][32] The jail houses pre-trial inmates and convicted persons who are serving sentences of no longer than two years less one day.

Chester is home to the Philadelphia Union of Major League Soccer; the team plays at Subaru Park, a venue located at the base of the Commodore Barry Bridge.

Delaware County is the traditional home of women's professional soccer in the Philadelphia area.

The Philadelphia Independence of Women's Professional Soccer succeeded the Charge and played at Widener University's Leslie Quick Stadium in 2011.

Delaware County has two physical geographic regions: the Piedmont and the Atlantic Coastal Plain, Most of the county has a humid subtropical climate (Cfa) while some higher northern areas have a hot-summer humid continental climate (Dfa.).