A 2015 small-mammal survey in the Boé region of Guinea-Bissau captured eight Macmillan's Grammomys. Its field guide describes a small, long-tailed, arboreal mouse.
The back is olive-brown to gray, the flanks are paler, and the underside is white to cream. The rump is reddish brown, rusty hairs edge the ears, and the tail is very long.
The survey guide describes it as arboreal. Eight animals were captured in the 2015 Boé survey, but the report does not give species-specific feeding behavior or activity times.
An extinction-risk assessment has not been added yet.