Mount Vernon is a city in Jefferson County, Illinois, United States, and its county seat. The population was 14,600 at the 2020 census. Mount V…Mount Vernon is a city in Jefferson County, Illinois, United States, and its county seat. The population was 14,600 at the 2020 census. Mount Vernon is the principal city of the Mount Vernon, Illinois micropolitan area, which includes all of Jefferson and Hamilton counties.
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