Middlesex, New Jersey. Middlesex is a borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 13,635 reflecting a decline of 82 (-0.6%) from the 13,717 counted in the 2000 Census, which had in turn increased by 662 (+5.1%) from the 13,055 counted in the 1990 Census.