Oxford Township was formed from portions of Greenwich Township on May 30, 1754, while the area was still part of Sussex County. It was incorporated on February 21, 1798, as one of New Jersey's initial group of townships by an act of the New Jersey Legislature. Mansfield Township became part of the newly formed Warren County on November 20, 1824. Over the centuries since its creation, portions of the township were taken to form Knowlton Township (February 23, 1763), Franklin Township, Harmony Township and Hope Township (all on April 8, 1839), Belvidere (April 7, 1845) and White Township (April 9, 1913).