Mount Vernon is a city in Linn County, Iowa, United States, adjacent to the city of Lisbon. The city's population was 3,390 when the 2000 census figures were released, but that number was later revised to 3,808 because the Census Bureau had incorrectly reported that 418 residents of a Cornell College dormitory in Mount Vernon lived in the nearby city of Bertram. A special census taken by the city in 2004 counted 4,171 residents. The population was 4,506 at the 2010 census.