Originally granted by Governor Benning Wentworth in 1761, Dorchester was named for Dorchester in Dorset, England. When the recipients failed to take up the grant, it was regranted in 1766, but also without success. Finally, it was regranted by Governor John Wentworth to 72 people on May 1, 1772, and settlement began soon thereafter. The first settlers were Benjamin Rice and Stephen Murch from Hanover, but originally from Connecticut.

When the first census of Dorchester was taken in 1790, there were 175 residents. By 1859, when the population reached 711, there were eleven sawmills, in addition to several clapboard and shingle mills. Charcoal was also manufactured here.

According to the United States Census Bureau, the town has a total area of 45.2 square miles (117 km²), of which 44.7 sq mi (116 km²) is land and 0.6 sq mi (1.6 km²) is water, comprising 1.22% of the town. It is drained by the South Branch Baker River and Indian River. The highest point in Dorchester is on its western boundary, where the elevation reaches 3,190 feet (970 m) above sea level, just east of the summit of Smarts Mountain. Dorchester lies within two watersheds — roughly the southwestern half of town is in the Connecticut River watershed and the northeastern half is in the Merrimack River watershed.

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