It runs for 77.530 miles (124.772 km) through the central region of the state connecting US Highway 10 (US 10) near North Bradley with M-72 in rural Crawford County near the community of Luzerne.

In between, the roadway connects several smaller communities while running through woodlands in both state and national forest areas.

Two segments of M-18 run along sections of county boundaries, and one part of the highway forms a component of the business loop for the village of Roscommon.

When the highway was first designated in the late 1910s, it extended farther south than it does today and ended well short of its current northern terminus.

M-18 begins about 20 miles (32 km) 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.

[3][4] The main entrance to Backus Creek State Game Area is located along this stretch of M-18 just south of I-75.

In the middle of Roscommon, BL I-75 turns northwest along Old 76 Road,[4] and M-18 continues northeasterly across a line of the Lake State Railway.

[8] No section of M-18 has been listed on the National Highway System,[9] a network of roads important to the country's economy, defense, and mobility.

[12][13] When the new alignment for US 27 was opened in early 1949, M-18 was re-extended north from Prudenville to Roscommon, where it turned northwesterly with M-76 to end at US 27 south of Grayling.