The area was first settled in 1774 by Nathaniel Segar from Newton, Massachusetts. Phineas Howard from Temple, New Hampshire purchased from Massachusetts in 1792 the unoccupied land in the township, including Howard Pond. The community became known as Howard's Gore. Combined with a portion of Bethel separated by the Androscoggin River, it then became Hanover, named by its German settlers for the House of Hanover. It was incorporated on February 14, 1843. Endowed with fine loam soil, farming became an early business. Indeed, Hanover was known for having some of the best intervale farms in the state. A variety of crops were grown, the chief being hay. The outlet of Howard Pond provided water power for industry. At Hanover Village, 12 mills were established. They produced flour, woolens, furniture, short and long lumber, sash, blinds, doors, dowels and rakes. Other goods included leather, boots and shoes. By the early 1950s, however, the last mills had declined and closed. More recently, Hanover has become site for both year-round homes and vacation properties.
According to the United State Census Bureau, the town has a total area of 7.56 square miles (19.58 km2), of which, 7.04 square miles (18.23 km2) of it is land and 0.52 square miles (1.35 km2) is water. Hanover is bounded by the Androscoggin River on the south and lies just southwest of Rumford on U.S. Route 2 and Maine State Route 5.