The northwestern garter snake is restricted to the northwestern corner of the state where it is most common in wet meadow, lacustrine, fresh emergent wetland and slow-moving riverine habitats, and in clearings in dense forests. It is also found in forests and grassy areas some distance from water. In the water it dives to the bottom and takes cover under banks or snags. Young are live-born and require a quiet, protected place for birth but nothing else is known about reproductive requirements.
Permanent water is probably a physiological as well as ecological requirement.
The northwestern garter snake is found in a variety of aquatic and semiaquatic habitats in close association with forests where slugs, earthworms and amphibian prey can be found. This snake is diurnal, actively foraging and basking near water throughout the day. Courtship has been observed from late March to mid-April. The young are live-born about 9 weeks later. Garter snakes are preyed upon by northern harriers and probably other raptors, and possibly herons.