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 anything of Jefferson and Hamilton counties.

Mt. Vernon was founded in 1817 by Zadok Casey, who was elected to the State Senate in 1822 and was elected Lieutenant Governor in 1833. He served in the U.S. Congress with 1833 and 1843. The town was named for George Washington’s plantation, Mount Vernon, which was named for Edward Vernon, a British naval hero.

When the town was founded, there was no road to it. Travelers had to gain there by either past the high ground from the north or crossing the swamps from the south. In the ahead of time 19th century the Goshen Road crossed Illinois in a northwesterly meting out from Old Shawneetown, Illinois to the Goshen Settlement, near what is now Edwardsville. This road was the main road in Illinois. When Mt. Vernon was first settled, the Goshen Road made a broad arc across Jefferson County, crossing Casey Creek and the huge Muddy north of Mt. Vernon, avoiding the swamps to the south, but bypassing Mt. Vernon. The road entered the county at its southeast corner. It passed through, or near, what are now Opdyke, East Salem, Idlewood, Dix and Walnut Hill. However, it was apparent to the to come settlers that the town would fail without roads. In 1820–1821, Ben Hood and Carter Wilkey built a bridge exceeding Casey Creek, to the southeast of town. This bridge was close the present bridge upon Illinois Route 142. A road was built from there northwest, over pitch that is now impassable, toward the outdated cemetery in back the advanced Bethel Cemetery. Deep cuts through the archaic cemetery attest to the location of the road. From there the road probably followed highly developed Route 37 into town, somewhere shifting from 10th Street upon west to 12th Street.

After the welcome capital was moved to Vandalia in 1819, it became apparent that a road to Vandalia was needed. A party was sent out to the northwest to mark the road. In 1823, Thomas D. Minor and William Maxwell built the “Vandalia Road”, now called the “Old Centralia Road.” It runs northwest out of Mt. Vernon to Walnut Hill. Although legend says that this road is crooked because of the drunken declare of the surveyors, the passageway is probably just the natural lane of a buccaneer road with the terrain. After the bridge and the Vandalia Road were built, Mt. Vernon was “on the map.” The bridge across Casey Creek and the Vandalia Road provided a much shorter lane across Jefferson County than the indigenous Goshen Road. The further Goshen Road soon captured most of the traffic, and Mt. Vernon became an important End on the road west.