|Jefferson County Illinois Cities/Towns - Belle Rive, Bluford, Bonnie, Dix, Ina, Mount Vernon (County Seat), Nason, Scheller, Spring Garden, Texico, Waltonville, Woodlawn
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Jefferson County is a county located in the southern part of the U.S. state of Illinois. As of 2000, the population was 40,045. The county seat is Mount Vernon.
Jefferson County was organized in 1819. It was named in honor of Thomas Jefferson, principle draftsman of the Northwest Ordinance, among other things.