Lake County is a county in the northeastern corner of the U.S. state of Illinois. According to the 2010 census, it has a population of 703,462, making it the third-most populous county in Illinois. Its county seat is Waukegan. Originally part of McHenry County, the townships composing Lake County were carved out into a separate county in 1839. Lake County is one of the collar counties of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area.