| M.L. Kapp Generating Station | Clinton | IA |
| Monsanto | Muscatine | IA |
| Monsanto | Davenport | IA |
| Moore Engineering | Waterloo | IA |
| Morgan Colonial Heating & | Des Moines | IA |
| Morningside College | Sioux City | IA |
| Morningside/East High School | Sioux City | IA |
| Mt. Arbor Nurseries | Shenandoah | IA |
| Municipal Light & Power | Hartley | IA |
| Municipal Light & Power | Cedar Falls | IA |
| Municipal Light & Power | Pella | IA |
| Municipal Light & Power | Spencer | IA |
| Municipal Light & Power | Atlantic | IA |
| Municipal Light & Water Plant | Atlantic | IA |
| Murray Iron Works | Burlington | IA |
| Muscatine Lighting | Muscatine | IA |
| Muscatine Municiple Electric Plant | Muscatine | IA |
| Muscatine Power Department | Muscatine | IA |
| Muscatine Power & Light | Muscatine | IA |
| Muscatine Power & Water | Muscatine | IA |
| M.W. Kellogg | Fort Dodge | IA |
| Mystic Milling | Sioux City | IA |
| National Animal Disease | Ames | IA |
| National By Products | Clinton | IA |
| National By Products | Des Moines | IA |
| National Carbon | Red Oak | IA |
| National Gypsum | Fort Dodge | IA |
| National Motor Inn | Des Moines | IA |
| National Starch & Chemical | Des Moines | IA |
| National Travelers Life Insurance Building | Des Moines | IA |
| Natural Gas Manufacturing Plant | Fort Dodge | IA |
| Natural Gas & Pipeline | Emerson | IA |
| Natural Gas & Pipeline | Truro | IA |
| Natural Gas & Pipeline | Harper | IA |
| Neal Smith Federal Office Building | Des Moines | IA |
| New High School | Maquoketa | IA |
| Newton & Northwestern Railroad | Fraser | IA |
| Nichols Wire & Aluminum | Davenport | IA |
| Nordyke & Marmon | Indianapolis | IA |
| North American Manufacturing | Sioux City | IA |
| Northern Indiana Gas & Electric | Sioux City | IA |
| Northern Natural Gas | Ventura | IA |
| Northern Natural Gas | Mason City | IA |
| Northern Sugar | Mason City | IA |
| North High School | Sioux City | IA |
| North States Port Cement | Mason City | IA |
| Northwestern Bell Telephone Building | Davenport | IA |
| Northwestern Bell Telephone Building | Des Moines | IA |
| Northwestern Portland Cement | Mason City | IA |
| Northwest National Bank | Sioux City | IA |
