It runs in north–south in southeast Wisconsin from the Illinois Border near Lake Geneva to south of East Troy.

After 3.5 miles (5.6 km) of straight road, WIS 120 turns east, then curves north to bypass downtown Lake Geneva.

4.5 miles (7.2 km) later, it meets WIS 50 for a short concurrency (picking up CTH-H along the way).

While CTH-H turns south toward downtown, WIS 120 turns north to meet the western terminus of WIS 36 and the White River State Trail in unincorporated Springfield after 2.5 miles (4.0 km) of rural road.

It continues as such up to and including the WIS 50 overlap and the roughly 1 mile (1.6 km) section of freeway it shares with US 12 on the east side of Lake Geneva.