According to the U.S. Census Bureau, the county has a total area of 552 square miles (1,430 km2), of which 537 square miles (1,390 km2) is land and 15 square miles (39 km2) (2.7%) is water. It is the third-smallest county in New Hampshire by area.
The courthouse square of Sullivan County originally was the site of a curious V-shaped mound, pointing to the northwest, elevated about 15 feet at the highest point. When leveling the mound, excavators found three Indian skeletons. Stones which had been placed around the skeletons were used as foundation stones for the jail.
The second courthouse for Sullivan County occupied the site of this mound. Major John McCullough constructed the building in 1857-58 at a cost of $5,000. The cupola was added later. The 1877 Atlas. This was the first brick structure in Milan.
Sullivan County: While putting in the railroad through Sullivan County Skeletons were found along this route. These native skeletons were of huge proportions they were described by a local indian named Phillip in which the skeletons are in the possession of Dr. Robbins family.