The highway meets US 10 near Sears after crossing the Pere Marquette State Trail. M-66 crosses the Muskegon River near a separate 9 Mile Road in Osceola County. It meets both M-115 and M-61 (16 Mile Road) south of Marion. The highway continues north and crosses the Great Lakes Central Railroad for the first time in Marion, before entering Missaukee County. The trunkline then turns westward on Stoney Corners Road toward McBain through farm land. In town it runs along Maple Street and then runs north toward Lake City on Morey Road. South of the Lake City, M-55 runs concurrently with M-66 by Missaukee Golf Course and into town along the eastern shore of Lake Missaukee. North of town, M-55 splits off to the east on Houghton Lake Road, and M-66 continues north to an intersection with the eastern terminus of M-42 in a rural forest. M-66 leaves Morey Road and follows Pioneer Road to the county line.