 Portions of the township were taken to form Fieldsboro (March 7, 1850, within township; became fully independent c. 1894), Bordentown Township (March 8, 1852) and Florence Township (March 7, 1872).The township was named for Mansfield, England.According to the United States Census Bureau, the township had a total area of 21.907 square miles (56.740 km2), including 21.740 square miles (56.307 km2) of land and 0.167 square miles (0.432 km2) of water (0.76%).