Joannes Gennadius (1844-1932) was a diplomat by trade and a bibliophile by obsession, the kind of collector who pursued rare Greek books across decades and continents. While he was attending the Washington Naval Treaty conference, American scholars caught wind of his collection and his frustration, and they took interest in founding a dedicated library in Greece. The American School of Classical Studies at Athens became its steward. With financial backing from the Carnegie Corporation, a building rose to house the 26,000 volumes Gennadius donated. He named it not for himself but for his father, George Gennadius (1784-1854), an educator who had helped shape Greek learning in the years after independence.

The New York firm of Van Pelt and Thompson designed the Gennadeion in the shape of a classical temple, complete with gardens, set on the green slopes below Lycabettus on Souidias Street. The architects John Van Pelt and W. Stuart Thompson gave the library a marble facade that would not look out of place among the ancient ruins their patrons studied. It opened on 23 April 1926. Inside, the collection had room to grow, and grow it did, from 26,000 volumes to more than 110,000. The library is one of two run by the American School of Classical Studies, alongside the Blegen Library, and together they make the school's Athens campus one of the richest scholarly archives in the eastern Mediterranean.

Among the Gennadeion's rarest holdings is a collection devoted to Lord Byron, the English poet who died at Messolonghi in 1824 fighting for Greek independence. The library preserves his portrait and the laurel wreath, entwined with wildflowers, that the people of Messolonghi offered in his memory. It holds books Byron himself once owned, including two copies of a 1640 edition of the Iliad translated into demotic Greek. And it guards one of the rarest pieces of all: a four-line verse Byron wrote in 1818 to mark the birth of a son to the English consul in Venice, printed in perhaps ten copies or fewer. To Greeks, Byron is no foreign curiosity. He is a hero who died for their freedom, and the Gennadeion keeps his memory the way a family keeps a portrait on the wall.

The library did not freeze in 1926. Since 1999 it has been expanded and modernized, with air conditioning, a new auditorium, and an east wing, all inaugurated in 2005 along with fresh offices and storage for the Gennadeion archives. Much of the funding came from American supporters, with the collector and philanthropist Lloyd Cotsen among the leading benefactors. Some of the original Gennadius material, including his personal scrapbooks and ephemera, has been digitized and placed online, so that a collector's lifelong gathering can now travel anywhere. The man who once watched his books vanish into anonymity finally got what he wanted: a home where every volume is known, catalogued, and kept.