Middlesex County is a county located in the south central part of the U.S. state of Connecticut. As of the 2010 census, the population was 165,676. The county was created in May 1785 from portions of Hartford County and New London County.Middlesex County is included in the Hartford-West Hartford-East Hartford, CT Metropolitan Statistical Area.As is the case with all eight of Connecticut's counties, there is now no county government, and no county seat.