It runs north–south in north central Wisconsin from a junction with US Highway 8 (US 8) near Cavour in central Forest County to the Michigan state line and its connection with M-189 at the Brule River to approximately four miles north of Tipler in Florence County.

Along its route, WIS 139 serves as the main access route to Long Lake and serves parts of the Nicolet side of the Chequamegon-Nicolet National Forest.

Starting at US 8 near Cavour, WIS 139 traverses northward through multiple forests.

It passes through Newald, Popple River, and Long Lake before reaching WIS 70.

After leaving the concurrency, WIS 139 travels northward for more than a mile (1.6 km) before crossing the Michigan state line above the Brule River.