Ammi B. Young designed the Georgetown Custom House and Post Office in the Renaissance style of architecture so that it resembled an Italian palace. The structure was built in 1857 out of granite. Two shallow sets of steps were also constructed of granite. The steps lead to over sized wooden doors that open to a huge open space. Custom Houses were established by Congress to serve as official ports of entry for foreign passengers and cargo. The second floor of the building was used as offices for customs and the Georgetown city offices. The basement became the holding area for arriving cargo to be inspected before it was released. A post office was established on the first floor and is still operational today. The customs operation was moved out of the building in 1967. In 1971 the Georgetown Customhouse and Post Office was designated on the National Registor of Historic Places.
The stately interior has wood floors throughout. Gold chandeliers hang from the ceiling. Cherry and maple wood were used as decorative accents around the post office windows. Floor to ceiling columns are designed to look like marble columns. This building was one of the first Custom Houses and Post Offices built. The post office is the longest operating post office in the United States.