| | Madison County | IA | 19121 | | Des Moines-West Des
Moines, IA 19780 | Des Moines-West Des
Moines-Ames, IA 218 | Outlying | metro county (urban) | nonspecialized | https://maps.google.com/?q=MadisonCounty+IA |
| | Mahaska County | IA | 19123 | | Oskaloosa, IA 36820 | Des Moines-West Des
Moines-Ames, IA 218 | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MahaskaCounty+IA |
| | Marion County | IA | 19125 | | Pella, IA 37800 | Des Moines-West Des
Moines-Ames, IA 218 | Central | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=MarionCounty+IA |
| | Marshall County | IA | 19127 | | Marshalltown, IA 32260 | | Central | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=MarshallCounty+IA |
| | Mills County | IA | 19129 | | Omaha, NE-IA 36540 | Omaha-Fremont, NE-IA 420 | Outlying | metro county (urban) | federal/state
government-dependent | https://maps.google.com/?q=MillsCounty+IA |
| | Mitchell County | IA | 19131 | | | | | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=MitchellCounty+IA |
| | Monona County | IA | 19133 | | | | | nonmetro county (rural) | farm-dependent | https://maps.google.com/?q=MononaCounty+IA |
| | Monroe County | IA | 19135 | | | | | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=MonroeCounty+IA |
| | Montgomery County | IA | 19137 | | | | | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=MontgomeryCounty+IA |
| | Muscatine County | IA | 19139 | | Muscatine, IA 34700 | Davenport-Moline, IA-IL 209 | Central | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=MuscatineCounty+IA |