Adults of this medium-sized hummingbird species are 11-12 cm in length. Males average 7.2 g, females, 6.8 g. The longish black bill is slightly decurved. The tail in both sexes has dark central feathers. In adult males, the outer tail feathers are deep magenta to wine purple tipped with black. In females and juvenile males, the outer tail feathers are broadly banded in magenta and iridescent dark blue, with narrow white tips on the outer 3-4 feathers.

The adult male has glossy bright green upperparts. His throat and chest have a relatively narrow matte black central area, bordered with blue-green. The flanks are bright green, and the black of the chest tapers onto the belly.

Females and immature males have bronze-green upperparts and largely white underparts with a dark central stripe that changes from black at the chin to blue-green on the throat. Immature birds of both sexes usually show some gray or buff feather edges on the head and wings and are mottled with cinnamon to rusty brown along the edges of the white breast-belly stripe. Immature females have less extensive magenta in the outer tail feathers than adult females or immature males.

The call of the Green-breasted Mango is a high-pitched tsup, and the song is a buzzing kazick-kazee-kazick-kazee-kazick-kazee-kazick-kazee.

This species is very similar to the closely related Black-throated Mango. Although the male Green-breasted Mango has less extensive black on the underparts, this and other plumage differences are not always easy to confirm in the field because the birds appear all-black. The females of the two species can be almost inseparable, although Green-breasted has more extensively coppery upperpart tones than its relative.

This species breeds from eastern and southern Mexico south through Central America, including some near-shore islands, to Costa Rica. In western and central Panama, it is replaced by the possibly conspecific Veraguan Mango Anthracothorax veraguensis. Disjunct populations occur along the northern coast of South America from extreme northeastern Colombia through northernmost Venezuela, in the upper Cauca River Valley of southwestern Colombia, and on the coastal slope of southwestern Ecuador and extreme northwestern Peru.

The species is partially migratory, occupying its breeding range in northeastern Mexico (southwestern Tamaulipas and eastern San Luis Potosí to southern Veracruz and extreme western Tabasco) from late February through September. Other movements are poorly understood, but the wide separation of populations in South America suggests a species-wide propensity to travel and/or a more continuous distribution during periods when South America's climate was warmer and drier.

Young birds are responsible for the majority of occurrences in the United States. The first Green-breasted Mango documented north of Mexico was photographed in coastal Texas in September 1988. The species has since become an increasingly frequent vagrant and extremely rare resident in the lower Rio Grande Valley of southern Texas.

Only three individuals have been documented in the U.S. outside Texas, all visiting feeders at private homes: a juvenile male in Concord, North Carolina in November 2000, a juvenile male in Beloit, Wisconsin first identified in September 2007, and a juvenile bird, possibly a male, in Dublin, Georgia, in October 2007. Neighbors of the Wisconsin bird's hosts report that the bird had been visiting their feeders since late July. On 5 November 2007, the Beloit Green-breasted Mango was captured to prevent its death from winter weather. Against the advice of hummingbird experts, the bird was transferred from the Wildlife Rehabilitation Center of the Wisconsin Humane Society to the Brookfield Zoo, near Chicago, which elected to confine the bird permanently with five hummingbirds already in its Perching Bird House.

The species inhabits tropical deciduous forest, open landscapes with scattered large trees, orchards, gardens, and cultivated areas, but its distribution is spotty and often localized.