Originally comprised of land granted to the officers and soldiers of a 1690 expedition to Canada, Ashburnham was first settled by European colonists in 1736 and called the Plantation of Dorchester-Canada until it was incorporated in 1765.

Once the second largest manufacturer of chairs in the world, Ashburnham is home to the internationally recognized Cushing Academy which opened in 1875 as one of the first coeducational boarding schools on the East Coast.

Its extensive natural resource network includes both state and town forests with a third of its undeveloped land designated as open space. Ashburnham has seven lakes, numerous ponds, two monadnocks—Mount Watatic and Little Watatic Mountain—several hills, and the convergence of two major New England hiking trails, the Midstate and the Wapack. It is the source of the Souhegan River flowing north, the Millers River flowing west, the North Branch of the Nashua via Phillips Brook flowing east and the Whitman River flowing south.

There are no interstates, limited-access highways, or stoplights within the town.