Biodiversity across elevational gradients generally follows patterns, the evolutionary origins of which are debated. We trapped small non-volant mammals across an elevational gradient on Mount (Mt.) Kinabalu (4,101 m) and Mt. Tambuyukon (2,579 m), two neighboring mountains in Borneo, Malaysia. We also included visual records and camera trap data from Mt. Tambuyukon. On Mt. Tambuyukon we trapped a total of 299 individuals from 23 species in 6,187 trap nights (4.8% success rate). For Mt. Kinabalu we trapped a total 213 animals from 19 species, in 2,044 trap nights, a 10.4% success rate. We documented the highest diversity in the low elevations for both mountains, unlike previous less complete surveys which supported a mid-elevation diversity bulge on Mt. Kinabalu. Species richness decreased gradually towards the highlands to a more even community with different species (high turnover), less rich but with the highest levels of endemism. These patterns suggest that an interplay of topography and climatic history of the region were drivers of the diversity gradient, in addition to standing climatic and spatial hypothesis.