[5] In 1968 Moss became the Executive Director of The Athenaeum of Philadelphia, a member-supported library founded in 1814 and housed in a National Historic Landmark building near Independence Hall.
During his four-decade tenure there he restored and expanded the building[6] and reorganized the nearly moribund institution as an independent research library specializing in American architecture prior to 1930 and nineteenth-century material culture.
[7] Under his direction the research collections in architecture and Victorian-era design rapidly expanded, including the acquisition of the archives of numerous major American architects ranging from Thomas Ustick Walter (1804-1887) to Paul Philippe Cret (1876-1945).
[11] Moss also proved to be an aggressive collector with remarkable fund raising ability, particularly for the acquisition, conservation, proper housing, and exhibition of architectural records, securing major grants from national, state, and local foundations for those purposes.
[12] In 1976 Moss launched a publication series to reprint rare Victorian design sources from the Athenæum collection, beginning with Exterior Decoration: A Treatise on the Artistic Use of Colors in the Ornamentation of Buildings originally published in 1885 by the Devoe Paint Company www.1754Paint.com complete with large color plates and authentic paint samples.
His works at the Athenaeum of Philadelphia since 1968 include restoration of the institution's building, and amassing a significant collection of historic architectural drawings and photographs for its library.