Little black shags have sleek, all-black plumage with a dark green glossy sheen on the upper wings apparent in good light. They have dark facial skin, a thin black to dark grey bill and black legs and feet. The iris is bright green, and there are bluish-green tubercles around the eye. Small nuptial plumes develop during courtship but these are lost during breeding.

Males and females are identical. Juveniles have dark brownish-black plumage, and chicks have yellowish bills. Little black shags are smaller than black shags and they are often seen in flocks of a few birds to about 100, occasionally more. They breed in colonies in trees overhanging water, and sometimes on the ground on small islands.

Voice: little black shags are quiet except when breeding, when males utter a variety of throaty croaks and whistles.

Similar species: many juvenile little shags are completely dark, but they differ from little black shags in having stubby yellow bills and relatively longer tails. Black shags are much larger, and usually have yellow facial skin; breeding adults have a white flank patch that is never present on little black shags. The little black shag has a much brighter green eye than either species, if seen at sufficiently close range.