The Library of Pantainos was built by Titus Flavius Pantainos. It really was a library complex, which included the library proper as well as the two attached stoas running along the front and side of the building. These stoas contained small shops and a cult room dedicated to Trajan, who was emperor at the time Pantainos built the library.
There is no known reference to this library in any of the surviving Greek texts. The strangely-shaped building is known to be a library because of two things. One, the library’s rules were found in situ. Two, the lintel with the library’s dedication was found in the Herulian Wall, built from the spoila from the agora and the library itself. The Herulian Wall rests on part of the library, the portico facing the agora.
It is from the inscription on the door lintel that we know what we know about Pantainos. The building, the porticos and the books were given to the city of Athens in the name of the emperor Trajan and Athena; the gift was from Titus Flavius Pantainos, his son, Flavius Menandros (named after his grandfather) and his daughter, Flavia Secundille.
Research has identified a Pantainos of Gargettos as having served as eponymous archon after 102, which was around the time the library was built. Since “Pantainos” is an uncommon name, it has been assumed that this archon was the same person who built the library. Therefore, he was either a citizen or made a citizen of the city, perhaps for his donation of the library. He and his family probably also had Roman citizenship, and they were wealthy. Only wealthy men could serve in the eponymous archon position. Also, Pantainos was a philosopher like his father, Flavius Menandros. However, it is not known if Pantainos headed a philosophical school, or was an amateur philosopher.
According to Arthur W. Parsons, “Pantainos” appears only twice in Attic prosopography. The first Pantainos is of interest. He was the father of Thucydides of Gargettos, who was an adversary of Perikles. He is probably an ancestor of the library’s donors.
Seventy-five years later in Alexandria, a Pantainos again appears. He is mentioned by Eusebius and Clement; they talk of him and his great influence. He had been a Stoic but by the time of Commodus, he was heading the Catechetical School there. Philip says that he was an Athenian, but neither Eusebius nor Clement mention where he is from; Parsons tentatively assumes Philip’s statement as true and therefore concludes that this Pantainos, who headed the first Christian school in Alexandria, was the grandson of the Pantainos who gifted Athens with a library.
Parsons, Arthur W. “A Family of Philosophers at Athens and Alexandria.” Hesperia Supplements, v. 8: Commemorative Studies in Honor of Theodore Leslie Shear (1949), p. 268-272, 462.