This snake is found in meadows and at the edges of clearings in forests. It prefers areas with dense vegetation but, when basking, can be found in open areas or on talus slopes. It occurs in wooded areas on the floor of the Willamette Valley and has been found in the Rogue Valley. This garter snake is commonly found in suburban areas and city parks.
The Northwestern garter snake feeds mainly on slugs and earthworms, but also takes insects, small salamanders, frogs, fish, small mammals, and possibly nestlings of ground nesting birds.