Hackettstown is a town in Warren County, in the U.S. state of New Jersey. It is perhaps best known as the home to the US headquarters of Mars, Inc. As of the 2020 United States census, the town's population was 10,248, an increase of 524 (+5.4%) from the 2010 census count of 9,724, which in turn reflected a decline of 679 (−6.5%) from the 10,403 counted in the 2000 census.