After his appointment as Deputy Keeper of the Record Office in 1861 Hardy edited, for the Rolls Series, "A Descriptive Catalogue of MSS relating to the History of Great Britain and Ireland" (1862–71), the "Registrum Palatinum Dunelmense" (1873–1878), and a "Syllabus in English of Rymer's Foedera" (1869); and he also commenced for the same series "Lestorie des Engles solum Geffrei Gaimar." Besides these works he made reports on the documents preserved at Venice relating to English history, and on the collection of papers at the Bodleian Library.