Middlesex County is a county in the south central part of the U.S. state of Connecticut. As of the 2020 census, the population was 164,245. The county was created in May 1785 from portions of Hartford County and New London County.

Hartford County is a county located in the north central part of the U.S. state of Connecticut. According to the 2020 census, the population was 899,498, making it the second-most populous county in Connecticut. Hartford County contains the city of Hartford, the state capital of Connecticut and the county's most populous city, with 121,054 residents at the 2020 census. Hartford County is included in the Hartford-East Hartford-Middletown metropolitan statistical area.