M-66 continues northward through Barry County on Capital Avenue which becomes 9 Mile Road north of Baseline Road. The highway passes through Assyria before meeting M-79, with which it has a short concurrency, in Nashville. On the north side of the village, M-66 crosses the Thornapple River and continues north through mixed rural forest land and farm fields. Near Woodland, M-66 joins M-43 and the two run to the northeast and along the Barry–Eaton county line. M-43/M-66 meets M-50 at a four-way intersection southwest of Lake Odessa near Woodbury, and M-43 turns east leaving M-66 in favor of a concurrency with M-50. M-66 crosses a rail line of CSX Transportation and the county line on State Road. It meets I-96 in a rural southern part of the Ionia County south of Ionia. On the south edge of town, the highway passes the county airport and curves to the northeast becoming Dexter Street. While entering downtown Ionia, the trunkline crosses the Grand River and the abandoned mainline of the Grand Rapids Eastern Railroad near the county fairgrounds. M-66 turns west along M-21 (Lincoln Avenue) for two blocks before turning back to the north along State Street. The trunkline runs through the northern part of the county and meets M-44's eastern terminus near Woodard Lake.