Middletown is a city located in Butler and Warren counties in the southwestern part of the U.S. state of Ohio. 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 population of Middletown as of the 2010 census was 48,694. It is part of the Cincinnati-Middletown Metropolitan Statistical Area as defined by the U.S. Census Bureau.The city was the home of AK Steel Holding Corporation, a major steel works founded in 1900 until offices were moved to West Chester Township, Ohio in 2007, but AK Steel's factory still resides in Middletown. Middletown contains a small municipal airport known as Hook Field,, but is no longer served by commercial airlines, only general aviation. A regional campus of Miami University is located in Middletown. In 1957, Middletown was designated as an All-America City.