Mountain gorillas are descendants of ancestral monkeys and apes found in Africa and Arabia during the start of the Oligocene epoch (34-24 million years ago). The fossil record provides evidence of the hominoid primates (apes) found in east Africa about 18–22 million years ago. The fossil record of the area where mountain gorillas live is particularly poor and so its evolutionary history is not clear. It was about 9 million years ago that the group of primates that were to evolve into gorillas split from their common ancestor with humans and chimps; this is when the genus Gorilla emerged.

It is not certain what this early relative of the gorilla was, but it is traced back to the early ape Proconsul Africanus. Mountain gorillas have been isolated from eastern lowland gorillas for about 400,000 years and these two taxa separated from their western counterparts approximately 2 million years ago. There has been considerable and as yet unresolved debate over the classification of mountain gorillas. The genus was first referenced as Troglodytes in 1847, but renamed to Gorilla in 1852.

It was not until 1967 that the taxonomist Colin Groves proposed that all gorillas be regarded as one species (Gorilla gorilla) with three sub-species Gorilla gorilla gorilla (western lowland gorilla), Gorilla gorilla graueri (lowland gorillas found west of the Virunga) and Gorilla gorilla beringei (mountain gorillas including, Gorilla beringei found in the Virunga and Bwindi). In 2003 after a review they were divided into two species (Gorilla gorilla and Gorilla beringei) by The World Conservation Union (IUCN).