Mucuna sloanei is a large liana. The leaves are compound and alternate, with ovate leaflets measuring 6–12 cm long and 4–8 cm wide. The surfaces are covered with irritating yellowish-brown hairs, and the lower surfaces are silky strigose. The leaflet margins are entire, and the leaves are petiolate. The stipules are caducous, and the leaflets are stipellate. The flowers are arranged in elongate, pendulous pseudoracemes, scattered along the upper part of the rachis. The flowers are papilionaceous, with deltate, campanulate, and bilabiate calyx lobes. The upper calyx lobes are 6–7 mm long, and the lowest is 11–13 mm long. The corolla is bright yellow tinged with red, 4–6 cm long, and showy, with the standard petal shorter than the other petals. The wings are incurved and stiffened at the apex, and the keel is straight to slightly curved upward. There are ten stamens, with the upper stamen distinct and the other nine connate into a tube. The anthers are dimorphic, with five larger subbasifixed anthers alternating with five shorter anthers. The ovary is superior, with a single apical style, few ovules, a filiform style that is glabrous apically, and a small terminal stigma. The fruit is a flat pod, about 10 cm long and 4–5 cm wide, with several transverse lamellae. The surface is covered with irritating yellowish-brown hairs, especially persistent near the lamellae. The seeds are few, dark brown, subglobose, about 2.5 cm in diameter, with the hilum extending about a quarter of the circumference. It is found sprawling over vegetation at low elevations on O‘ahu, Maui, and Hawai‘i, but it is known to persist only around Ulalena Hill between 500–700 meters on East Maui.