The Masai Mara National Reserve (MMNR) covers some 1530km² in south-western Kenya. It is the northern-most section of the Mara-Serengeti ecosystem, covering some 25,000 km². It is bounded by the Serengeti Park to the south, the Siria escarpment to the west and Maasai pastoral ranches to the north, east and west. Rainfall in the ecosystem increases markedly along a south-east-north-west gradient, varies in space and time, and is markedly bimodal. The Sand, Talek and Mara are the major rivers draining the reserve. Shrubs and trees fringe most drainage lines and cover hillslopes and hilltops.

The terrain of the reserve is primarily open grassland, with seasonal riverlets. In the south-east region are clumps of the distinctive acacia tree. The western border is the Esoit Oloololo Escarpment of the Rift Valley, and wildlife tends to be most concentrated here, as the swampy ground means that access to water is always good and tourist disruption is minimal. The easternmost border is 224 km from Nairobi, and hence it is the eastern regions which are most visited by tourists.

Wildebeest, zebra and Thomson's gazelle migrate into and occupy the Mara reserve from the Serengeti plains to the south and Loita plains in the pastoral ranches to the north-east from July to October or later. Herds of all three species are also resident in the reserve.

All members of the "Big Five" are found in the Masai Mara, although the population of black rhinoceros is severely threatened, with a population of only 37 recorded in 2000. Hippopotami are found in large groups in the Masai Mara and Talek rivers. Cheetah are also found, although their numbers are also threatened, chiefly due to tourist disruption of their daytime hunting. As mentioned above, the plains between the Mara river and the Esoit Oloololo Escarpment are probably the best area for game viewing, in particular regarding lion and cheetah.

As in the Serengeti, the wildebeest are the dominant inhabitants of the Masai Mara, and their numbers are estimated in the millions. Around July of each year these ungainly animals migrate in a vast ensemble north from the Serengeti plains in search of fresh pasture, and return to the south around October. The Great Migration is one of the most impressive natural events worldwide, involving an immensity of herbivores some 1,300,000 wildebeests, 360,000 Thomson's gazelles, and 191,000 zebras. These numerous migrants are followed along their annual, circular route by a block of hungry predators, most notably lions and hyena.

Numerous other antelopes can be found, including Thomson's and Grant's gazelles, impalas, topis and Coke's hartebeests. Large herds of zebra are found through the reserve. The plains are also home to the distinctive Masai giraffe as well as the common giraffe. The large Roan antelope and the nocturnal bat-eared fox, rarely present elsewhere in Kenya, can be seen within the reserve borders. The Masai Mara is a major research centre for the spotted hyena. Additionally, over 450 species of birdlife have been identified in the park, including vultures, marabou storks, secretary birds, hornbills, crowned cranes, ostriches, long-crested eagles, and African pygmy-falcons.