Situated in a hilly, rocky, forested area, and with 26 lakes and ponds, Washington is a picturesque resort area. It is home to Pillsbury State Park. Ashuelot Pond is in the southwest, and part of Highland Lake is in the south. Lovewell Mountain, elevation 2,496 feet (761 m) above sea level, is the highest point in town. The Washington Town Common, elevation 1,507 feet (459 m), is the highest town center in the state. Washington is bordered by Goshen to the northwest, Newbury and Bradford to the northeast, Hillsborough and Windsor to the southeast, Stoddard to the south, Marlow to the southwest, and Lempster to the west.
Granted in 1735 by Colonial Governor Jonathan Belcher of Massachusetts, the town was one of the fort towns designated to protect the colonies from Indian attack, named "Monadnock Number 8". In 1751, the town was granted by Governor Benning Wentworth as "New Concord". As the grant was never settled, the charter was revoked. Reuben Kidder of New Ipswich acquired the grant, naming it "Camden". The first settlers arrived in 1768 and built log houses. By 1773, the community had 132 inhabitants. On December 13, 1776, the newly established American revolutionary government incorporated the town as "Washington", after George Washington — the first named in his honor.
By 1830, the population had grown to 1,135. Using water power from the streams, mills manufactured lumber, barrel staves, shingles, chair parts, bobbins, whip sockets, hosiery, bricks and washboards. Gristmills ground grain. Sheep farms, producing wool and mutton, were an important business. In 1840, there were 200 farms. Tubbs Union Academy was founded in 1849, and although it did not last long, the school once enrolled over 100 students from New Hampshire and beyond. The first Seventh-day Adventist church was established in Washington in 1862. In 1886, the town produced 53 tons of maple sugar.
The railroad era brought tourists, and hotels were built on the lakes and ponds; however, with 19th century migration to the Midwest, the town's population dwindled. As of 1960, it contained only 162 inhabitants. Washington has grown since, as its scenic beauty attracts tourists and retirees. The town contains significant examples of early architecture, including the Town Hall, built as a meetinghouse in 1787, the Congregational Church built in 1840, and the first Seventh-day Adventist Church built circa 1843.