The northern grass mouse is a medium-sized species. The fur on the upper parts of the body is a dark chestnut grizzled with black. The underparts are paler, with the hairs having dark bases and yellowish or pale grey tips. There is a bare ring of skin round the eye and there are usually grey-tipped hairs on the chin and throat. The ears are sparsely covered with rusty brown hairs. The tail is a uniform dark brown with a scattering of hairs and no tuft at the tip. The upper surfaces of the feet have dark ochre fur and there are tufts of hair by the nails. The first and fifth digits of the hind feet are much shorter than the other three.
The northern grass mouse is active both day and night. The diet consists of green plant material and seeds. Females reach sexual maturity at 2.7 months and breeding takes place throughout the year with a peak in May and June. The average litter size is five and there are three or four litters per year.