Separated from the nearest church and school by the Monadnock hills, the residents successfully petitioned to form a new town in 1791, using the name "Greenfield" to highlight the area's level, fertile ground.

Established by Harry Gregg, the facility on Crotched Mountain originally treated for polio, cerebral palsy, spina bifida and other physical and neurological disabilities.

Greenfield is home to the Yankee Siege, considered the most powerful (current) trebuchet in the world, which has participated in the annual World Championship Punkin' Chunkin' Contest in Sussex County, Delaware, since 2004.

[1] The southeastern part of Greenfield is drained by Stony Brook, flowing southeast to the Souhegan River in Wilton, while the western part, including the town center, is drained by the Contoocook River and its tributary Otter Brook; the Contoocook forms the northwestern border of the town, where it passes through Powder Mill Pond.

The northeastern corner of the town is drained by Rand Brook, a tributary of the South Branch of the Piscataquog River.

[4] North Pack Monadnock, elevation 2,276 feet (694 m) above sea level, is the northernmost summit of the Wapack Range and the highest point in Greenfield, located in the extreme southern part of the town.