| | McHenry County | IL | 17111 | Chicago-Naperville-Schaumburg,
IL 16984 | Chicago-Naperville-Elgin,
IL-IN 16980 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=McHenryCounty+IL |
| | McLean County | IL | 17113 | | Bloomington, IL 14010 | Bloomington-Pontiac, IL 145 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=McLeanCounty+IL |
| | Macoupin County | IL | 17117 | | St. Louis, MO-IL 41180 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Outlying | metro county (urban) | nonspecialized | https://maps.google.com/?q=MacoupinCounty+IL |
| | Madison County | IL | 17119 | | St. Louis, MO-IL 41180 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=MadisonCounty+IL |
| | Marion County | IL | 17121 | | Centralia, IL 16460 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MarionCounty+IL |
| | Mason County | IL | 17125 | | | | | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MasonCounty+IL |
| | Mercer County | IL | 17131 | | Davenport-Moline-Rock
Island, IA-IL 19340 | Davenport-Moline, IA-IL 209 | Outlying | metro county (urban) | nonspecialized | https://maps.google.com/?q=MercerCounty+IL |
| | Monroe County | IL | 17133 | | St. Louis, MO-IL 41180 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=MonroeCounty+IL |
| | Montgomery County | IL | 17135 | | | | | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MontgomeryCounty+IL |
| | Morgan County | IL | 17137 | | Jacksonville, IL 27300 | Springfield-Jacksonville-Lincoln,
IL 522 | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MorganCounty+IL |