Philadelphia, Bucks, and Chester were the three counties created by William Penn on August 24, 1682, in the colonial-era Province of Pennsylvania.

[5][6] At the time, Chester County's borders were Philadelphia County to the north, the ill-defined western edge of the colony, located approximately at the Susquehanna River to the west, the Delaware River to the east, and Delaware and Maryland to the south.

Easy transportation allowed workers to commute to urban jobs, and the rise of the suburbs followed.

During the American Revolutionary War, the Battle of Brandywine was fought in the southeastern part of the county.

Many of the soils are fertile, rich loam as much as twenty-four inches thick; together with the temperate climate, this was long a major agricultural area.

[11] Because of its proximity to Philadelphia, Chester County has seen large waves of development over the past half-century due to suburbanization.

[12] Lanchester Landfill, located on the border of Chester and Lancaster Counties, captures methane which is sold for renewable natural gas credits, and piped to seven local businesses.

This reduces the county's methane emissions, and provides an alternative to fracking for shale gas.

[13] In addition, several companies have their headquarters or a major presence in the county including Bentley Systems, EBS Healthcare, Main Line Health, Lavazza North America (formerly Mars Drinks), Depuy Synthes (part of Johnson & Johnson), Metabo, QVC, Hankin Group, Axalta Coating Systems, CTDI, Pactiv, Ricoh Americas, Blinding Edge Pictures, J.G.

The region was originally occupied by the Lenni Lenape people, who greeted European settlers in the seventeenth century with amity and kindness.

From the late 19th to early 20th century, the industrial areas of the region, such as Coatesville, attracted immigrants and job seekers from Germany and Ireland, Eastern Europe, Italy, and the American rural South, with both black and white migrants coming north.

the fastest-growing county in the Delaware Valley; it is one of the fastest growing in the entire Northeastern section of the United States.

In keeping with its colonial history, Chester County is home to a number of historic Quaker buildings, including Birmingham, Birmingham Orthodox, Bradford, Caln, Old Kennett, Parkersville, Schuylkill, Westtown, and Uwchlan meeting houses.

The First Presbyterian Church of West Chester, Coventryville United Methodist Church, which is part of the Coventryville Historic District, and Beth Israel Congregation of Chester County, a Conservative synagogue in Coatesville, a site of Eastern European immigration in the 20th century, are located in the county.

In 2009, with a smaller turnout, Republican candidates swept all county-row offices, winning with an average margin of 20%.

Republican candidates John Brown and John Rafferty carried Chester County, though both lost their races for Auditor General and Attorney General, respectively (Rafferty, a State Senator whose district includes northern Chester County, carried the county by a slim margin of 50 votes[22]) Emphasizing its Democratic shift even further, Joe Biden defeated Donald Trump by 17.1 points in Chester County in the 2020 election; Trump's percentage of votes was the lowest for any Republican since 1912.

[24] In both the 2021 and 2023 elections, Democrats followed up with another sweep of the county row offices, along with retaining their majority on the Board of Commissioners.

[25] Chester County is administered by a three-person Board of Commissioners who serve four-year terms.

The commissioners have selective policy-making authority to provide certain local services and facilities on a county-wide basis.

Under Pennsylvania law, there are four types of incorporated municipalities: cities, boroughs, townships, and, in at most two cases, towns.

The following cities, boroughs and townships are located in Chester County: Census-designated places are unincorporated communities designated by the U.S. Census Bureau for the purposes of compiling demographic data.

[44] † county seat Chester County has four distinct seasons and has a hot-summer humid continental climate (Dfa) except for some far southern lowlands and areas along the Schuylkill River which have a humid subtropical climate (Cfa).

Of the 104 drug overdoses recorded by the coroner, an estimated 77 percent involved the presence of fentanyl.