M-18 begins about northeast of Mount Pleasant at the North Bradley interchange on US 10 in rural Midland County. The highway runs due north through woodlands that are a part of the Au Sable State Forest and into Gladwin County. The trunkline enters the community of Beaverton and turns northwest to cross the section of the Tobacco River known as Ross Lake. Continuing northwesterly, the road runs along a section of the lake and then turns back due northerly. North of Beaverton, M-18 runs through some farm fields as it approaches the county seat of Gladwin. The highway runs through the southwestern outskirts of town near a golf course as it approaches a junction with M-61. M-18 merges with, and runs concurrently along, M-61 eastward into town on Cedar Avenue. At Silverleaf Street, M-18 turns back north near the airport. M-18 continues northward out of town back into farm country.