A Neoclassical architect, Mills generally followed the principle, enunciated by Jefferson, that antique classical architectural forms best befitted a reincarnation of ancient republics—i.e., the new United States. He used classical forms with originality. Mills's more than 50 major works included colleges, prisons, hospitals, houses, canals, bridges, and breakwaters. His best-known structures are the Treasury (built 1836–42) and the Old Patent Office (built 1836–40; later modified; now part of the Smithsonian Institution) in Washington, D.C.; the wings of Independence Hall in Philadelphia (1807); and the monuments to George Washington in Baltimore, Md. (designed 1814, erected 1815–29), and Washington, D.C. (designed 1836, completed 1884).