According to Euripides, Ion was the illegitimate child of Creusa, the daughter of the Athenian king Erechtheus and wife of Xuthus, by the god Apollo. According to the earliest known mythological accounts, including the Hesiodic Catalogue of Women, Ion was instead the legitimate son of Creusa and Xuthus.

The traditions surrounding Ion differ sharply between authors and regions: the Athenian tragedian Euripides dramatizes his divine paternity by Apollo, while the inhabitants of the northern Peloponnese and of Attica each preserved their own accounts of his kingship and his role in the war against Eleusis.

One story of Ion appears in the tragedy Ion by Euripides. Apollo visited Creusa in a cave below the Propylaea, where she conceived Ion. When the princess gave birth she abandoned the child in the same cave, but Apollo asked Hermes to take Ion from his cradle. He was saved, raised, and educated by a priestess of the Delphic Oracle. When he had grown, Xuthus and Creusa came to consult the oracle about obtaining an heir; the answer was that the first human being Xuthus met upon leaving the temple should be his son. Xuthus met Ion and recognized him as his son, although Apollo was in fact giving Ion to him as an adoptive son. Thinking the boy was her husband's son by another woman, Creusa presented Ion with a cup filled with poisonous dragon's blood.

Her plot was discovered when Ion poured a libation before drinking and a pigeon that drank from it died. Creusa fled to an altar, and Ion dragged her away to kill her, but a priestess intervened, revealed the truth, and showed that Ion was Creusa's own son. Mother and son were reconciled, though Xuthus remained uninformed; he was satisfied after receiving a promise that he would father Dorus and Achaeus.

The inhabitants of Aegialus, on the northern coast of the Peloponnese, were likewise Ionians but preserved a different tradition. Here Ion was the son of Xuthus who, after expulsion from Thessaly or Attica, was brought to the region during the reign of King Selinus. After his father's death, Ion prepared to march against the Aegialeans, but Selinus offered him his only child Helice in marriage and adoption as his successor, and Ion accepted. Upon Selinus's death he succeeded to the throne. He founded the city of Helice in Aegialus, naming it after his wife and making it the capital, and he called the inhabitants Ionians after himself. This was not a renaming but an addition, so that they became known as the Aegialian Ionians. By Helice, Ion fathered Bura, the eponym of the city Bura. Later he led an expedition against Eleusis with Athenian help and was killed in battle near Eleusis.

Other traditions represent Ion as king of Athens between the reigns of Erechtheus and Cecrops II. The Athenians called upon him during their war with the Eleusinians; he conquered Eumolpus and became king of Athens. He fathered four sons — Geleon (Teleon), Aegicores, Argades, and Hoples — after whom he divided the Athenians into four classes or tribes: the Hopletes (Hoplites), Teleonites (Geleontes), Aegicoreis, and Argadeis (Ergadeis). After his death he was buried at Potamus.

According to some accounts, Ion fathered Ellops, the founder of Ellopia, and possibly also Aïclus (Aiklos) and Cothus (Kothos), who founded the Euboean towns of Eretria and Cerinthus respectively.

Ion was believed to have founded the Ionians, a primary Greek tribe. He has often been identified with Javan, mentioned in the Hebrew Bible as the ancestor of the Greek people, though in the Bible Javan is a son of Noah's son Japheth. The earlier Greek form of the name was *Ἰάϝων ("Iáwōn"), which, with the loss of the digamma, became Ἰάων (Iáōn) or in the plural Iáones, as seen in epic poetry. Dionysius Periegetes mentions a river in Arcadia called Iaon, which is further alluded to in connection with Hesiod and the Hymns of Callimachus.