| L.R. Metal Treating | Stratford | CT |
| Luders Marine Construction, Stamford | Stamford | CT |
| M. A. Feinberg | Hartford | CT |
| Magnetic D S D | East Hartford | CT |
| Majestic Laundry | New Haven | CT |
| Malleable Iron Fittings | Branford | CT |
| Manchester Hospital | Manchester | CT |
| Manell | West Haven | CT |
| Manning, Maxwell & Moore | Stratford | CT |
| Manning, Maxwell & Moore | Bridgeport | CT |
| Mansfield State Hospital | Mansfield | CT |
| Mansfield Training Center | Mansfield | CT |
| Maple Street School | Vernon | CT |
| Marden & Wasserman | Hartford | CT |
| Marino Plumbing & Heating | Middletown | CT |
| Marn Plumbing & Heating | Waterbury | CT |
| Mary Immaculate Academy | New Britain | CT |
| Masonic Temple | Bridgeport | CT |
| Maxim Munitions | New Haven | CT |
| Maxim Silencer | Hartford | CT |
| Max Stein | Willimantic | CT |
| Meadow Hill Housing | East Hartford | CT |
| Meriden Gas Light | Meriden | CT |
| Meriden Molded Plastics | Meriden | CT |
| Merritt W. Burwell | New Haven | CT |
| Metallurgical Processing | New Britain | CT |
| Metalurgical Processing | New Britain | CT |
| Meublein | Hartford | CT |
| Mianus Manufacturing | Cos Cob | CT |
| Michael Hoffman Fuel | Bridgeport | CT |
| Middlesex Chemical | Chester | CT |
| Middlesex Hospital | Middletown | CT |
| Middletown Electric Light | Middletown | CT |
| Middletown Power Plant | Middletown | CT |
| Middletown Station Hartford Electric | Middletown | CT |
| Miles Laboratories | Orange | CT |
| Miles Laboratories | West Haven | CT |
| Milford Rivet & Machine C | Milford | CT |
| Milford Steam Power & Building | Milford | CT |
| Mills Memorial Apartments | Meriden | CT |
| Millstone Nuclear Power Plant | Waterford | CT |
| Millstone Nuclear Power Plant | New London | CT |
| Milner & Company | Moosup | CT |
| Mj Daly & Sons | Bridgeport | CT |
| M.J. Daly & Sons | Waterbury | CT |
| Modern Plumbing & Heating | Meriden | CT |
| Mohigan Hotel | New London | CT |
| Montville Power Plant | Montville | CT |
| Moore-McCormack Lines | Stamford | CT |
| Moran Towing & Transportation | Greenwich | CT |
