The Luzon montane forest mouse is a large, thickset species with a tail that is about the same length as the body. The soft, thick dorsal fur is dark brown, while the ventral side of the body is a cream white. The hind feet are partly brown in colour on the dorsal side, but otherwise white. The tail is brown on the dorsal side, and a cream white ventrally. The animal has a large skull with a square-shaped neurocranium. Some of its skull characteristics are so special that they set the animal apart from most every other species of Apomys. One specimen has a head-torso-length of 143 mm, a tail length of 144 mm and a hind feet length of 34 mm. The specimens captured by Sanborn on Mount Data have a mean skull length of 39.2 mm (37.0 to 40.6 mm with a standard deviation of 1.1 mm). The Luzon montane forest mouse's holotype has a skull length of 39.9 mm; that of A. major has a length of 38.4 mm.