The dataset represents occurrence records for mammal species in the Tana River Basin (TRB) ecosystem as well as a few outliers from the Aberdare National Park. The data were mainly sourced from published literature and the accessions register of mammal collections maintained by the Mammalogy Section, National Museums of Kenya (NMK). A few additional records were gleaned from unpublished grey literature as well as sightings and reports by ecotourism websites and lodges operating in TRB. Existing records were supplemented by fieldwork in selected areas to help in ground-truthing and filling data gaps. A total of 935 occurrence records represented by 16 orders, 46 families, 124 genera and 183 mammal species are presented here. Sites frequently visited in the past including Mt. Kenya and Meru National Parks had the largest number of mammal observations, while less visited sites such as Mumoni Hill Forest had very few records. The TRB is important for the protection of nine mammals endemic to Kenya including Cercocebus galeritus (Tana River Mangabey), Tachyoryctes rex (King Root-rat), Tachyoryctes spalacinus (Embi African Root-rat), Grammomys gigas (Giant Thicket Rat), Hylomyscus endorobae (Endorobo Wood Mouse), Otomys orestes (Afroalpine Vlei Rat), Crocidura fumosa (Smoky White-toothed Shrew), Surdisorex norae (Aberdare Mole-shrew) and Surdisorex polulus (Mount Kenya Mole-shrew). The ecosystem is also important for the conservation of three endemic mammal sub-species: Colobus guerza kikuyuensis (Mount Kenya Guereza), Cercopithecus mitis albotorquatus (Pousargues’s Monkey) and Perodicticus potto stockleyi (Mount Kenya Potto). Other species of conservation concern found in TRB are two Critically Endangered (CR) species namely, Beatragus hunteri (Hirola) and Diceros bicornis (Black Rhinoceros) as well seven endangered (EN) mammal species (Procolobus rufomitratus (Eastern Red Colobus), Cercocebus galeritus (Tana River Mangabey), Lycaon pictus (African Wild Dog), Equus grevyi (Grévy’s Zebra), Redunca fulvorufula (Mountain Reedbuck), Oryx beisa (Beisa Oryx) and Grammomys gigas (Giant Thicket Rat). This information underscores the importance of the entire TRB ecosystem for the conservation of mammal biodiversity.