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is a city located near the center of Jefferson County, Illinois, in the United States. In the 2000 census, the city's reported population was 16,269 people.
Mount Vernon is the principal city of the Mount Vernon Micropolitan Statistical Area, which includes all of Jefferson and Hamilton counties.
According to the United States Census Bureau, the town has a total area of 11.6 square miles (30.1 km²), of which, 11.5 square miles (29.8 km²) of it is land and 0.1 square miles (0.3 km²) of it (0.86%) is water.
Mt. Vernon is located on high ground between Casey Creek and the Big Muddy River, which join south of the town in what is now Rend Lake. In pre-settlement times the area around these waterways was a swamp, a heavily forested area that was waist-deep in water during much of the winter and during wet summers. Mt. Vernon was thus often surrounded by water and swamp on three sides.
High ground was located to the north of Mt. Vernon. A ridge ran between the Big Muddy River and Casey Creek north toward what is now Dix.
There are high places west and east of Mt. Vernon from which one can see the town as a forested point of high ground jutting out into the bottoms. These high places are: from the west, near the Woodlawn interchange off Interstate 64 and from the east, on Old Fairfield Road near Summersville School, with the highest point located on Old Route 15 right before it merges with New Route 15, near Bluford.