Hackettstown is a town in Warren County, New Jersey, United States. As of the 2010 United States Census, the town's population was 9,724, reflecting a decline of 679 from the 10,403 counted in the 2000 Census, which had in turn increased by 2,283 from the 8,120 counted in the 1990 Census. The town is located in the easternmost region of the Lehigh Valley.Hackettstown was incorporated as a town by an act of the New Jersey Legislature on March 9, 1853, from portions of Independence Township. Portions of territory were exchanged with Mansfield Township in 1857, 1860, 1872 and 1875.Hackettstown houses the headquarters of Mars Chocolate USA, the American division of Mars, Incorporated, makers of Milky Way, Mars, M&M's, Twix and Snickers candy bars, as well as pet foods, human foods and non-confectionery snack foods.It is believed that Hackettstown was named after Samuel Hackett, an early settler and large landowner.