Millville is a city in Cumberland County, in the U.S. state of New Jersey. As of the 2020 United States census, the city's population was 27,491, …Millville is a city in Cumberland County, in the U.S. state of New Jersey. As of the 2020 United States census, the city's population was 27,491, a decrease of 909 from the 2010 census count of 28,400, which in turn reflected an increase of 1,553 from the 26,847 counted in the 2000 census.