According to legend, Eleusis was founded by the hero Eleusis, son of Ogygos of Thebes, before the 15th century B.C. The town may have begun as a port town during Theban expansion. Regardless, excavations have revealed remains from the Middle Helladic II period, dating to the 18th-17th century B.C.

Many festivals of local and national significance were held here (Eleusinia, Thesmophoria, Haloa, Kalamaia). However, it was the Mysteries of Demeter which caused Eleusis to become so well-known, making it the most famous deme in Attica after Piraeus.

Little is known about when and how the cult of Demeter was brought to Eleusis. Apollodorus places the establishment of the cult in the 2nd half of the 15th century B.C. Egypt, Thrace, Thessaly, and Crete have all been suggested as places from where the cult was imported. In the Homeric Hymn to Demeter, a man named Eumolpos, later killed in the war against Athens, was reputed to be the first initiate.

When Eleusis came under the power of Athens, the cult and sanctuary of Demeter flourished and grew. By the 8th century B.C. it had become a pan-Hellenic cult. For centuries, the rituals attracted initiates from the entire Greek-speaking world, including men, women, slaves and foreigners.

What occurred within the inner sanctum of the Telesterion at Eleusis is entirely unknown, as the initiates were under pledge of secrecy not to divulge the revelation there given. Though the cult of Demeter operated in Eleusis for over 2,000 years with annual celebrations and countless initiates, this veil of secrecy still lies upon the Mysteries. Portions of the initiation were held in public and the record of these rites has been preserved, but the events that occurred at the sanctuary will remain shrouded in the densest mystery forever.