The sombre greenbul is 15–18 cm long, with mainly dull greyish olive-green plumage, paler on the underparts than above. There are variations in colour based on habitat range. Individuals located in southern Africa tend to be plain, olive-green. To the north, birds are yellower with greener upper parts. It has a white iris. The sexes are similar in plumage, but juveniles are even duller than the adult and have dark, grey eyes. There is no colour distinction among sexes. The subspecies A. i. hypoxanthus is much yellower below than the nominate subspecies.
The sombre greenbul is a vocal species, and the most typical call is a monotonous, ringing phrase starting with a penetrating single whistle weeeewee, followed by a jumbled chortle and ending off with a rather plaintive, drawn-out whistle willy. If agitated, a call of peeet peeet peeet repeatedly is likely to be heard.
The sombre greenbul is a common bird, which tends to stay hidden in foliage and is more often heard than seen. It is usually found in pairs or small groups foraging for insects, fruit and small snails.
