Leptomantis harrissoni is of medium size, with the female (up to 70 mm) larger than the male (> 50 mm). It inhabits lowland rainforests. It is a brown, rather unspectacular frog with fully developed dark hand webbing. The snout is angular and pointed. 

Little is known about this frog, probably because it spends most of its life high up in the trees. But occasionally we have seen them at heights of 1-2 m. Eggs are deposited in a foam nest that is attached to the bark above a tree hole or buttress phytothelm. 

Tadpoles of this species grow fairly large: up to 40 mm. The tail fin is fully developed and reaches the trunk-tail junction. The tail tip is moderately pointed. The body is broadly ovoid; the snout is short. The tadpole is dark in color and appear dark brown in the field (flash light photography makes them appear lighter).