The species' range is now known to extend across the mountain ranges of eastern Madagascar from the Tsaratanana Massif south to Andohahela, at above sea level. It occurs in montane forest, but also in degraded grassland, where it is among the first species to return after fires. At Ankaratra, where the species was recorded in 1929, it occurred in such grassland, where the nesomyine Brachyuromys betsileoensis was also found. The animal was again recorded at Ankaratra in 1996, this time in a heavily disturbed forest, where it occurred with Eliurus minor and the introduced black rat (Rattus rattus). At Andringitra, the animal was recorded in high montane forest together with six other nesomyines—Brachyuromys ramirohitra, Eliurus minor, Eliurus tanala, Eliurus webbi, Gymnuromys roberti, and Nesomys rufus—as well as the black rat. At Andohahela, Monticolomys was found at an altitude of in sclerophyllous forest. Its distribution corresponds to the High Mountain Domain, a region defined on the basis of plant distributions. This region is now discontinuous, but the High Mountain Domain habitat was continuous from mountain to mountain as recently as the early Holocene. Subfossil remains have been found in Mahajanga Province (northwestern Madagascar). Malagasy mountain mouse is morphologically uniform across its wide distribution.