It runs in a diagonal southwest–northeast direction across southeastern Wisconsin from Springfield which is north of Lake Geneva to Milwaukee.

[2] The road continues northeast, passing through Waterford where it connects with WIS 164, then to Wind Lake and Muskego[3] before heading to Franklin, where it runs concurrently with US Highway 45 (US 45) for a short distance, and crosses WIS 100.

The highway then follows Loomis Road through Greendale and Greenfield, where it intersects with Interstate 43 (I-43) and I-894 before terminating at WIS 241 in Milwaukee.

The road originally began in Lake Geneva, approximately four miles (6.4 km) south of its current terminus, and ended near downtown Milwaukee.

Later, in 1987–1988, WIS 120 was extended north from Lake Geneva via WIS 36 to Springfield, and then north along County Trunk Highway G (CTH-G) toward East Troy, placing the end of the highway at its present location.