Middletown is a city in Jefferson County, Kentucky,United States. It was chartered as a city in 1797. The population was 5,744 at the 2000 census. When the government of Jefferson County merged with the city of Louisville, Kentucky in 2003, citizens of Middletown also became citizens of the new Louisville Metro, however, none of the incorporated places in Jefferson County were affected. Eastern High School is located in Middletown.