Starting at the corner of Main Street and Chicago Drive, M-121 runs northeasterly out of Zeeland on Chicago Drive roughly parallel to I-196. A CSX railroad line also parallels the road to the northwest. The trunkline is a four-lane divided highway bordered by farms. Passing the Hudsonville Fair Grounds, Chicago Drive enters Hudsonville as the main street downtown. The highway continues to the northeast out of town through suburban residential areas to Jenison, where once again Chicago Drive is bordered by commercial properties. Through this area, the roadway is divided, utilizing Michigan lefts. The M-121 designation ends at the interchange with I-196 just across the Kent County line in Grandville.