Vernon County is a county located in the western region of the U.S. state of Missouri, on the border with Kansas. As of the 2020 census, the population was 19,707. Its county seat is Nevada. The county was organized on February 27, 1855, considerably later than counties in the eastern part of the state. It was named for Colonel Miles Vernon, a state senator and veteran of the Battle of New Orleans. This area was part of the large historic territory of the Osage Nation of Native Americans.