Mount Vernon is a city in and the county seat of Lawrence County, Missouri, United States. As of the 2020 census, its population was 4,…Mount Vernon is a city in and the county seat of Lawrence County, Missouri, United States. As of the 2020 census, its population was 4,526. Mount Vernon was platted in 1845. The city was named after Mount Vernon, the estate of George Washington. A post office called Mount Vernon has been in operation since 1846.