Leptomantis angulirostris is a montane species that inhabits rocky mountain streams at elevations above 900 m a.s.l. The angular snout shape is characteristic and gives the species its name (though see also the similar L. penanorum). 

Males reach up to about 35 mm in snout–vent length, while females grow to around 50 mm. The body is moderately robust in males and more so in females. The fingers and toes end in large, round adhesive disks, especially prominent in females, which make these frogs excellent climbers. The angled snout connects to the eye via a sharp canthus. Webbing is present not only on the toes but also at the base of the fingers. The dorsal skin is smooth and lacks tubercles or warts, while the ventral skin is finely granular. Males are typically brown above, whereas females are green. The lower flanks may be yellowish, with dark speckling or marbling. In some populations, one or two bright white spots may be present on the upper lip. Typically the iris is coppery red around the pupil and bluish in the uppermost part. 

These frogs are unlikely to be seen during the day, but at night they perch on vegetation 2–4 m above the ground. Several individuals are often observed in a single spot, though they usually do not congregate in large numbers.

The tadpoles of this species reach a total length of up to 50 mm. They possess a cup-shaped suctorial mouth and can be distinguished by a few characteristic black spots on the tail, which they share with the tadpoles of L. penanorum. Tadpoles inhabit clear, cool mountain streams, where they use their mouth sucker to cling to rocks. They likely feed on algal and bacterial growth on rock surfaces. As with the adults, the tadpoles are more frequently observed at night.