Building 9 was constructed in 1839 in the Greek Revival style and was initially used as the Post Hospital.

[3][4][5] Ten years later, two wings for patient wards were built because the original structure was so poorly ventilated.

[4] In 1878–1879, Building 9 was converted to offices for the Military Division of the Atlantic and the Department of the East.

[3][4][5] The building was used for various Army commands until the 20th century, and by 1947, the structure was converted to four sets of officers' quarters.

[4][5] Building 9 is two-stories with a red-brick facade, raised above a high schist basement.