Green ash is a medium to large tree with a straight, tall trunk, high branches, and a rounded crown.
Leaves are opposite, compound, 8–12 inches long, with 7–9 lance-shaped leaflets, 4–6 inches long, finely toothed; upper surface dark green, dull, smooth; lower surface paler green, more or less hairy; leaflet stalks (at least the lower ones) narrowly winged. Leaves turn bright yellow in autumn.
Bark is brown, shallowly grooved, with narrow, flat-topped, interlacing ridges that form diamond or X-shape patterns.
Twigs are stout, rigid, gray, smooth.
Flowers April–May, male and female on separate trees; both male and female flowers in clusters, small, green, lacking petals.
Fruits August–September, in dense clusters to 8 inches long; fruit 1–2 inches long, a flattened, smooth, somewhat oarlike, winged seed, with the wing partially around the seed.