St. Charles County is a county in the eastern part of the U.S. state of Missouri. As of the 2010 census, the population was 360,485, making it Missouri's third-most populous county. Its county seat is St. Charles. The county was organized October 1, 1812 and named for Saint Charles Borromeo, an Italian cardinal. The county executive is Steve Ehlmann, since January 2007. - Source: Wikipedia