Illinois Route 142 is a major arterial road in southeastern Illinois. It runs from near the town center of Equality at Calhoun Street, a local road, north to Illinois Route 37 and the northern terminus of Illinois Route 148 in Mount Vernon. This is a distance of 55.06 miles (88.61 km).

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