The dorsum is brown with some weak marking; the ventral surface is immaculate or with some mottling.

[3][4][5] Mertensophryne usambarae is a terrestrial species that inhabits lowland forests not higher than 410 m (1,350 ft) above sea level.

It appears to be uncommon; i.e., three days of intensive survey effort in 2012 only revealed two specimens.

It is threatened by habitat loss caused by agricultural encroachment, wood extraction, and human settlement.

Because of the very small range and threats to the habitat, the International Union for Conservation of Nature (IUCN) assessed this species as "Critically Endangered" in 2016.