High Quality Content by WIKIPEDIA articles! Monroe Township is a Township in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the township population was 39,132, reflecting an increase of 11,133 (+39.8%) from the 27,999 counted in the 2000 Census, which had in turn increased by 5,744 (+25.8%) from the 22,255 counted in the 1990 Census.