| Code | Name | Region |
|---|---|---|
| USMTQ | Mattapoisett | MA |
| USKMJ | Mattaponi | VA |
| USWXJ | Mattawa | WA |
| USMVD | Maryville | MO |
| USMRL | Maryville | TN |
| US9DI | Masardis | ME |
| USMSA | Masaryktown | FL |
| USAOT | Mascot | TN |
| USMO5 | Mascotte | FL |
| USMSK | Mascoutah | IL |
| USUCH | Mashantucket | CT |
| USME7 | Mashpee | MA |
| USMWJ | Mason | MI |
| USUXE | Mason | OH |
| USQON | Mason | TN |
| USYAQ | Mason | TX |
| USMCW | Mason City | IA |
| USM2C | Mason City | IL |
| USONQ | Mason, Bayfield | WI |
| USQER | Masontown | PA |
| USVXI | Masontown | WV |
| USMI5 | Masonville | IA |
| USMPB | Maspeth | NY |
| USMRB | Martinsburg | WV |
| USIIE | Martinsville | IL |
| USMXS | Martinsville | IN |
| USMV8 | Martinsville | NJ |
| USMTV | Martinsville | VA |
| USAMV | Marvell | AR |
| USZVX | Marvin | TN |
| USMYT | Mary Esther | FL |
| USZJW | Maryland | LA |
| USZJV | Maryland City | MD |
| USMHY | Maryland Heights | MO |
| USMY4 | Maryneal | TX |
| USMYV | Marysville | CA |
| USMYE | Marysville | KS |
| USMVV | Marysville | MI |
| USMRV | Marysville | OH |
| USMYP | Marysville | PA |
| USMYS | Marysville | WA |
| USVLP | Maryville | IL |
| USYIB | Maryville | IL |
| USMFI | Marshfield | WI |
| USMHD | Marshville | NC |
| USMSG | Marsing | ID |
| US9MS | Marston | MO |
| USM4S | Marston | NC |
| USXJE | Martel | OH |
| USMVY | Martha's Vineyard | MA |