[2] The county was organized October 1, 1812, and named for Saint Charles Borromeo, an Italian cardinal.

St. Charles County is part of the St. Louis, MO-IL Metropolitan Statistical Area and contains many of the city's northwestern suburbs.

The St. Charles County Sheriff's Department (SCCSD) is responsible for court services and security, prisoner transport, civil process, and bailiffs.

Until the end of 2014, SCCSD was the primary law enforcement agency serving unincorporated areas of St. Charles County.

[citation needed] In May 2022, Ryan Keuhner, who was then an SCCSD deputy, shot and killed his neighbor's 3-year-old rescue dog with a pellet gun.

Video of the dog's owner confronting Keuhner was uploaded online, and later gained national coverage.

[26][27] In 2023, the county council issued a resolution aimed at library policies and staff online activities.

The resolution followed protests and comments at council meetings from residents who complained about a staff member wearing gender non-conforming clothes.

[28] The Republican Party predominantly controls politics at the local level in St. Charles County.

[29][30][31] St. Charles County is divided among eleven legislative districts in the Missouri State House of Representatives, all of which are held by Republicans.

St. Charles County is divided into three districts in the Missouri State Senate, each of which are represented by Republicans.

Missouri is represented in the U.S. Senate by Josh Hawley and Eric Schmitt, whose most recent election results from the county are included here.

Senator Rick Santorum (R-Pennsylvania) won the most votes in St. Charles County, with 56.29 percent.

Republican Former Governor Mitt Romney (R-Massachusetts) won the most votes in St. Charles County, with 37.72 percent.

Senator Hillary Clinton (D-New York) received a total of 23,611 votes, more than any candidate from either party in St. Charles County during the 2008 presidential primary.

Today the Westplex is made up of St. Charles, St. Peters, Weldon Spring, Cottleville, Dardenne Prairie, O'Fallon, Lake St. Louis, and Wentzville.