Metaneira is wife of Celeus of Eleusis and mother of Demophon, the child Demeter nurses. Many prose accounts also make her mother of Triptolemus; other genealogies give Triptolemus different parents (Eleusis, Oceanus and Gaia, Dysaules, and more). Pausanias identifies her as daughter of the Athenian Amphictyon. Daughters of the Eleusinian house appear under varying name-lists in the Homeric Hymn and in Pausanias.

During Demeter's search for Persephone, the goddess — disguised as the old woman Doso — was welcomed at Celeus' palace and given Demophon to nurse. By night Demeter anointed the child with ambrosia, breathed upon him, and laid him in the hearth-fire to burn away mortality. Metaneira saw the infant in the flames and screamed in fright, breaking the ritual.

Most sources say Demophon suffered no lasting harm but lost the chance of immortality; some later theories claimed the flames destroyed him. Demeter rebuked mortal folly and, forestalled in deifying the child, taught Triptolemus the arts of sowing and reaping. In Ovid's Fasti the nursed baby is named Triptolemus rather than Demophon — a minority identification against the usual adult or adolescent Triptolemus of the dragon-chariot.

Some mythological traditions tell that Metaneira's son Abas mocked Demeter and was turned into a lizard; others relate that punishment to Ascalabus, son of Misme. Through the hearth episode Metaneira becomes the human witness whose alarm redirects Demeter's gift from personal immortality for one child to cultivated grain and mystery rites for humankind.