MacMillan's shrew is found in Ethiopia. A 2020 genetic study suggested that its distribution may be limited to the forest zone of southwestern Ethiopia.
A 1980 paper described specimens then called C. luna macmillani as dark and medium-sized. This is an older description under a historical taxonomic name.
DNA sequences from specimens analyzed in a study published in 2020 grouped with known C. macmillani sequences. The authors suggested that its range may be limited to southwestern Ethiopia's forest zone.
The paper’s habitat table associates this species with Ethiopian montane tropical forest and humid Terminalia–Combretum woodland savanna with tall grasses. Its two specimens used for morphometric analysis were collected in 1998 in the Middle Godjeb Valley of southwestern Ethiopia.
In a 2016 comparison, the tail of C. afeworkbekelei—brown above and brownish gray below—is described as similar to this species. The two C. macmillani specimens in the table averaged 72.5 mm in head–body length and 56.0 mm in tail length (range 55.0–57.0 mm).
Four specimens examined in a study published in 2020 had very hairy tails, with long hairs covering 66–75% of the tail length.
An extinction-risk assessment has not been added yet.