Mount Vernon is a city in and the county seat of Posey County, Indiana, United States. Located in the state's far southwestern corner, within 15 miles (24 km) of both the southernmost or westernmost points, it is the westernmost city in the state. The southernmost is Rockport, located along the Ohio River about 40 miles (64 km) to the southeast.

Posey County is the southernmost, southwesternmost, and westernmost county in the U.S. state of Indiana. Its southern border is formed by the Ohio River, and its western border by the Wabash River, a tributary to the Ohio. As of 2020, the population was 25,222. The county seat is Mount Vernon.