For a period, Petropedetes newtonii was considered a junior synonym of this species, leading to a much broader distribution.

The two species are now considered distinct, but information on Petropedetes johnstoni sensu stricto is limited.

[2][4] The specific name johnstoni honours Harry Johnston, a British explorer and naturalist[5] who collected the holotype.

The male lacks the hypertrophied forearm and the keratinised spicules on the skin of the throat and on the basis of the arms seen in Petropedetes newtonii.

At the time, loss of its forest habitat due to agricultural development, logging and expanding human settlements was considered as the major threat.