Gennadius had started collecting books young. In 1867, for the
Exposition Universelle in Paris, he prepared an exhibit and
catalogue raisonné for all of the newspapers and periodicals published in Greece in 1866. In 1872, his personal library was not especially notable. The one "treasure" of his collection at that time was a first edition of the
Erotokritos. During the period 1874 to 1880, his collecting became more systematic, including not just books, but also drawings and prints on Turkish and Greek life and costume. In the 1880s, he "formed the 'grand design' that was to dominate his collecting for the rest of his life: to form a library that represented the creative genius of Greece at all periods, the influence of her arts and sciences upon the western world, and the impression created by her natural beauty upon the traveller", with the plan of donating it to the
National Library of Greece. In the meantime, he organized various activities in support of Greece. He arranged the donation of 6,000 books to the Parliamentary Library. He helped form the Greek Committee, with
Lord Rosebery as president. He helped found the
Society for the Promotion of Hellenic Studies. In 1922, he offered his collection of 24,000 volumes to the
American School of Classical Studies in Athens. A site for a library building was donated by the Greek government and funding for the building by the
Carnegie Corporation. Gennadius and his wife formally dedicated it on April 23, 1926. ==Honors==