The fur of the mountain gorilla, often thicker and longer than that of other gorilla species, enables them to live in colder temperatures. Gorillas can be identified by nose prints unique to each individual. Males, at a mean weight of 195 kg (430 lb) and upright standing height of 150 cm (59 in) usually weigh twice as much as the females, at a mean of 100 kg (220 lb) and a height of 130 cm (51 in). This subspecies is on average the second largest species of primate; only the Eastern Lowland Gorilla, the other subspecies of Eastern Gorilla, is larger. Adult males have more pronounced bony crests on the top and back of their skulls, giving their heads a more conical shape. These crests anchor the powerful temporalis muscles, which attach to the lower jaw (mandible). Adult females also have these crests, but they are less pronounced. Like all gorillas they feature dark brown eyes framed by a black ring around the iris.

Adult males are called silverbacks because a saddle of gray or silver-colored hair develops on their backs with age. The hair on their backs is shorter than on most other body parts, and their arm hair is especially long. Fully erect, males reach 1.9 m (6 ft 3 in) in height, with an arm span of 2.3 m (7 ft 7 in) and weigh 220 kg (490 lb). The tallest silverback recorded was a 1.94 m (6 ft 4 in) individual shot in Alimbongo, northern Kivu in May 1938. There’s an unconfirmed record of another individual, shot in 1932, that was 2.06 m (6 ft 9 in) tall. The heaviest was a 1.83 m (6 ft) silverback shot in Ambam, Cameron which weighed about 266 kg (590 lb).

The mountain gorilla is primarily terrestrial and quadrupedal. However, it will climb into fruiting trees if the branches can carry its weight, and it is capable of running bipedally up to 6 m (20 ft).] Like all great apes other than humans, its arms are longer than its legs. It moves by knuckle-walking (like the common chimpanzee, but unlike the bonobo and both orangutan species), supporting its weight on the backs of its curved fingers rather than its palms.

The mountain gorilla is diurnal, most active between 6:00 a.m. and 6:00 p.m. Many of these hours are spent eating, as large quantities of food are needed to sustain its massive bulk. It forages in early morning, rests during the late morning and around midday, and in the afternoon it forages again before resting at night. Each gorilla builds a nest from surrounding vegetation to sleep in, constructing a new one every evening. Only infants sleep in the same nest as their mothers. They leave their sleeping sites when the sun rises at around 6 am, except when it is cold and overcast; then they often stay longer in their nests.