Middletown is a city located in Butler and Warren counties in the southwestern part of the U.S. state of Ohio. The population as of the 2020 census was 50,987. It is part of the Cincinnati metropolitan area, located 29 miles (47 km) northeast of Cincinnati and 20 miles (32 km) southwest of Dayton. Formerly in Lemon, Turtlecreek, and Franklin townships, Middletown was incorporated by the Ohio General Assembly on February 11, 1833, and became a city in 1886. The city was the home of AK Steel Holding Corporation (formerly Armco), a major steel works founded in 1900. Although offices were moved to nearby West Chester Township in 2007, the AK Steel factory is still in Middletown. Middletown is also home to Hook Field Municipal Airport (airport code MWO), which was formerly served by commercial airlines but is currently only for general aviation. A regional campus of Miami University is located in Middletown. In 1957, Middletown was designated as an All-America City.