Middlesex County is a county located in New Jersey. It is part of the New Jersey Metropolitan Area. The center of population for New Jersey is located in Middlesex County, in the town of Milltown, just east of the New Jersey Turnpike. The county seat is New Brunswick.Middlesex County has a total area of approximately 323 square miles of which 96% is land and 4% water. As of the 2000 census, there were 750,162 people, 265,815 households, and 190,855 families residing in Middlesex County, NJ.