The site covers. It has a long, narrow shape, extending for about from north to south, and only from east to west. Mamiri lies on the boundary between the wet evergreen and moist evergreen forestzones. The terrain is hilly, with the hills strongly dissected by steep-sided deep valleys. These valleys become flooded during the rainy season, creating swampy habitats. The reserve lies at an elevation of about 128 m above sea level.
There are no recent estimates of western chimpanzee abundance in Ghana. The last estimate was made by Teleki, which estimated between 300 and 500 chimpanzees to be present in Ghana. The chimpanzee's presence was confirmed during field surveys in 2005 at this site. However, subsequent surveys in 2009 failed to confirm their survival..