Mount Vernon is a city in Knox County, Ohio, United States. The population was 16,990 at the 2010 census. It is the county seat of Knox County. The city is named after Mount Vernon, the plantation owned by George Washington. According to the United States Census Bureau, the city has a total area of 8.4 square miles (22 km2), of which, 8.4 square miles (22 km2) of it is land and 0.04 square miles (0.10 km2) of it (0.47%) is water. The city lies in the Glaciated Allegheny Plateau and is surrounded by rolling hills and valleys. The Kokosing River and U.S. Route 36 pass through the city.