According to the United States Census Bureau, the town has a total area of , of which is land and is water, comprising 2.09% of the town. Greenfield is drained by Stony Brook, Otter Brook and the Contoocook River; the town lies fully within the Merrimack River watershed. North Pack Monadnock Mountain, elevation above sea level, is the northernmost summit of the Wapack Range and the highest point in Greenfield.