According to the United States Census Bureau, the town has a total area of, of which is land and is water. The highest point in Chatham is the summit of South Baldface, with an elevation of above sea level. Sable Mountain, elevation, is in the west, and Robbins Ridge crosses the center of Chatham from east to west. Upper Kimball Pond is in the southeast, and Basin Pond in the northeast. Chatham lies fully within the Saco Riverwatershed. An article in Statistics and Gazetteer of New-Hampshire stated,
"Carter's Mountain rises so high between this town and Jackson as to prevent the opening of a highway, and the people of the respective towns, in their intercourse, are obliged to pass through a portion of Maine. Kiarsarge , in the south-west corner of the town, has...upon the summit...a commodious house for the accommodation of visitors. The views from this mountain are grand and very extensive. In the north-west corner is Baldface Mountain. Teams can be driven nearly to its summit. From the crest of this mountain the Tip-Top House on Mount Washington can be seen, also the ocean."
"Kiarsarge" and Baldface are now accessible only by hiking trails, and the summit of Kearsarge North has a lookout tower but no other structures. The views are still as extensive as before. Access to the town of Jackson continues to be partially through Maine, by taking Maine State Route 113 south to the Fryeburg area, then traveling west through Conway. Route 113, leading north, crosses into New Hampshire in the North Chatham part of town, then curves back into Maine to cross the mountains in Evans Notch and descend north to U.S. Route 2 in Gilead, Maine. Chatham is a pene-enclave, functionally accessible to neighboring New Hampshire towns only by traveling through Maine first.
As of the census of 2000, there were 260 people, 107 households, and 71 families residing in the town. The population density was 4.6 people per square mile. There were 253 housing units at an average density of 4.5 per square mile. The racial makeup of the town was 98.08% White, 1.15% from other races, and 0.77% from two or more races. There were 107 households, out of which 29.0% had children under the age of 18 living with them, 57.0% were married couples living together, 5.6% had a female householder with no husband present, and 33.6% were non-families. 27.1% of all households were made up of individuals, and 8.4% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.94. In the town, the population was spread out, with 24.6% under the age of 18, 4.2% from 18 to 24, 30.0% from 25 to 44, 30.4% from 45 to 64, and 10.8% who were 65 years of age or older. The median age was 41 years. For every 100 females, there were 106.3 males. For every 100 females age 18 and over, there were 106.3 males. The median income for a household in the town was $37,188, and the median income for a family was $40,156. Males had a median income of $22,188 versus $20,313 for females. The per capita income for the town was $15,317. About 13.3% of families and 15.4% of the population were below the poverty line, including 23.9% of those under the age of eighteen and none of those 65 or over.
