Robert Mills designed the General Post Office, completed in 1842. Thomas U. Walter oversaw the General Post Office's expansion from in 1855 to 1866. The General Post Office moved out in 1897. The General Land Office was a tenant from 1897 to 1917. The National Selective Service Board was a tenant in 1919. The Tariff Commission was a tenant from 1932 to 1988. The building was declared a National Historic Landmark in 1971.
In 2002, Michael Stanton Architecture in partnership with the Kimpton Group was selected by the General Services Administration to convert the building into a 184-room hotel, known as the Washington Monaco Hotel. The Washington Monaco Hotel was honored with the Washington DC Mayor's Award for Historic Preservation, the Business Week / Architectural Record Award, the GSA Heritage Award for Adaptive Use, and the GSA Heritage Award for Conservation and Restoration.