Eumolpus was most often described as the son of Poseidon (Neptune in Roman tradition) and Chione. An alternative genealogy made him instead the son of the god Apollo and the nymph Astycome. He is sometimes described as neither Greek, Thracian, nor Roman, but as Libyan and a native of North Africa, though his mother Chione is said to have been a Thracian princess.

He was the father of Immarados (also given as Immaradus or Ismarus) by the Oceanid Daeira. Other children associated with him in the tradition include the poet Musaeus and Ceryx, with a connection also drawn to Phorbas.

According to the mythographer Apollodorus, Chione, daughter of Boreas and the heroine Oreithyia, was secretly pregnant with Eumolpus by Poseidon. Fearing her father's reaction, she threw the newborn into the ocean after giving birth. Poseidon, however, looked after the child and brought him ashore in Ethiopia, where Benthesikyme, a daughter of Poseidon and Amphitrite, raised him as her own.

When he grew up, Eumolpus married one of Benthesikyme's two daughters by her Ethiopian husband. But Eumolpus loved a different daughter and made an attempt upon her chastity, and for this he was banished. He then went to Thrace with his son Ismarus (or Immaradus), who married the daughter of King Tegyrius.

Eumolpus was later discovered in a plot to overthrow King Tegyrius and was obliged to flee, going to Eleusis, where he formed a friendship with the Eleusinians and became one of the first priests of Demeter and one of the founders of the Eleusinian Mysteries. After his son's death, Tegyrius sent for Eumolpus to return to Thrace; they made peace, and Eumolpus inherited the Thracian kingdom.

The traditions about the Eleusinian war differ greatly. According to some accounts, the Eleusinians under Eumolpus attacked the Athenians under Erechtheus but were defeated, and Eumolpus, together with his two sons Phorbas and Immaradus, was slain.

An alternative tradition, recorded by Pausanias, holds that in the battle between the Eleusinians and the Athenians it was Erechtheus and Immaradus who fell, and that peace was then concluded on the condition that the Eleusinians should in other respects be subject to Athens but should alone retain the celebration of their mysteries. According to yet another version, after Erechtheus killed Eumolpus, Poseidon asked Zeus to avenge his son's death, and either Zeus killed Erechtheus with a lightning bolt or Poseidon made the earth open up and swallow him.

Although Eleusis lost the war with Athens, the Eumolpidae and the Kerykes, two families of priests of Demeter, continued to conduct the Eleusinian Mysteries.

Mythology regards Eumolpus as the founder of the Eleusinian Mysteries and as the first priest of Demeter and Dionysus; the goddess herself was said to have taught the sacred rites to him, Triptolemus, Diocles, and Celeus. He is sometimes described as having invented the cultivation of the vine and of fruit trees in general.

Eumolpus was an excellent musician and singer who played the aulos and the lyre, and he won a musical contest at the funeral games of Pelias. As an ancient priestly bard, poems and writings on the mysteries were later fabricated and circulated under his name. The legends also connected him with Heracles, whom he is said to have instructed in music or initiated into the mysteries.

Concerning his offspring, Diogenes Laertius made Eumolpus the father of Musaeus, while according to Philochorus he was the father of the legendary poet Musaeus by the lunar goddess Selene.

Scholars have identified the fifth-century BCE bronze statue known as Riace B as a possible representation of Eumolpus. The fingers of the well-preserved figure indicate that it originally carried a bow and arrow, weaponry typical of Thracian warriors.