Leptomantis harrissoni is a medium-sized treefrog, with females reaching up to 70 mm in snout–vent length (SVL) and males exceeding 50 mm. It inhabits lowland rainforests.

This species is generally brown and rather plain in appearance, though it can often show reddish tones on the back. The hands are notable for their fully developed dark webbing, with the three outer fingers webbed all the way to the adhesive discs. The tips of all fingers and toes expand into large discs. The snout is relatively long, angular, sharply edged, and pointed. White or cream colored spots can be present or absent on the upper lip under the eye. The skin lacks large tubercles or warts and is mostly finely granular. The iris is reddish around the pupil, transitioning to gray toward the outer edge.

Little is known about the species, likely because it spends most of its life high in the forest canopy. Occasionally, individuals descend to heights of just one or two meters, particularly when a suitable tree hole is nearby and available for reproduction. Eggs are deposited in foam nests attached to bark above tree holes or buttress phytotelmata. It is possible, though not yet proven, that females return to these sites to lay additional eggs, which are then consumed by the tadpoles already present. Tadpoles have been found with eggs in their gut. This facultative or perhaps obligatory oophagy could be a strategy so that more tadpoles can manage to survive in a tree hole than its volume would normally support. Further research is needed to confirm this speculation.

The tadpoles of L. harrissoni are relatively large, exceeding 40 mm in total length. The tail fin is well developed and extends to the junction of the trunk and tail, with a moderately pointed tip. The body is broadly ovoid and the snout is short. Tadpoles are very dark, appearing almost black in the field, although flash photography makes them appear lighter.