The Mason house, also known as “Quality Hill,” is one of the largest of the late eighteenth-century houses that grace Prospect Street, home to Georgetown's upper class. Its first occupant, John Thomson Mason, came from a prominent family with substantial landholdings in Maryland and Virginia, and this house provided him with an appropriate setting for socializing with the leaders of the young nation.
Like many of its contemporaries in Georgetown, the design of the Mason house was in part determined by the nature of the building materials. The house is fashioned of brick (now painted) laid out in Flemish bond. Decorative detailing includes keystone window lintels, a molded water table, a modillion cornice, and dormer windows. A carved wooden door surround barely adds a third dimension to the planar wall surfaces. During a major restoration of 1942–1944, the arch in the center hall, salvaged from the Francis Scott Key House, was added, as was flooring in the vestibule, reportedly from a historic house on Capitol Hill.