The western terminus of State Route 166 is at a T-intersection with U.S. Route 6 just over 3 miles (4.8 km) northeast of Chardon.

Its eastern terminus is at State Route 534 about 5 miles (8.0 km) west of Rock Creek.

State Route 166 starts in Hambden, traveling northeast through the northwest corner of Montville Township, passing by Curtis Airport.

After it crosses into Thompson Township, it curves to face the east.

In turn, State Route 528 was extended northward, replacing what was the north-south leg of State Route 166.