Cyanea mannii is a branched shrub growing 1.5 to 3 meters tall. Its leaves are simple and alternate, with narrowly elliptic to lanceolate blades measuring 12–21 cm in length and 2.5–5 cm in width. The leaf apex is acuminate, and the base ranges from obtuse to cuneate. Upper leaf surfaces are green and glabrous, while the lower surfaces are pale green, either glabrous or pubescent. Leaf margins are callose–serrulate. Petioles range from 2.2 to 10 cm long, and stipules are absent. The flowers are bisexual (perfect) and arranged in axillary racemes, composed of 6 to 12 flowers. The peduncles are 20–35 mm long, and the pedicels are 8–12 mm long. The hypanthium is broadly obconical, measuring 4–6 mm in length and 3–5 mm in width. The calyx has five triangular lobes, each measuring 3–5 mm in length and 2–3 mm in width, which are either distinct or rarely connate, with acute apices. The calyx tube is adnate to the ovary. The corolla is zygomorphic (bilaterally symmetric), purplish, 30–35 mm long and 3–4 mm wide, glabrous, with a suberect tube that is dorsally cleft to about the middle and spreading lobes. There are five stamens, alternate with the corolla lobes and connate into a glabrous staminal column. Anthers are glabrous except for the lower two, which have apical tufts of white hairs. They are dithecal, opening by longitudinal slits, and may be either coherent (separating after anthesis) or connate to form a tube through which pollen is shed. Filaments are distinct or connate above and are attached to either the epigynous nectary disk or the base of the corolla, and rarely adnate to the corolla tube. The ovary is inferior and 2-celled, with axile ovule placentation. The stigma is 2-lobed, initially appressed and nonreceptive, while the style grows through the anther tube and becomes receptive as the lobes spread apart after pollen release. The fruit is a berry. It is found in mesic to wet forests between 1,000-1,080 meters.