| 1 | MAR | March Helicopters Ltd. | Callsign: March, | GB | AVCODES |
| 2 | MAR | Air Mediterranean S.A. | IATA: MV, Callsign: Hellasmed, IATA Prefix: 012, Commenced operations on 01 November 2017. Suspended all operations on 18 January 2018. Re-started operations in February 2018 operating charter flights only. | GR | AVCODES |
| 3 | MAR | Air Mediterranean S.A. | IATA: MV, Callsign: Hellasmed, Commenced operations on 01 November 2017. Suspended all operations on 18 January 2018. Re-started operations in February 2018 operating charter flights only. | GR | AVCODES |
| 4 | MAR | March Helicopters | | | ICAO1 |
| 5 | MAR | Missouri and Arkansas Ry. Co. | 7/1935-4/1938 | | NAKINA |
| 6 | MAR | Missouri and Arkansas Ry. Co. | Apr-38 | | NRHS |
| 7 | MAR | *MISSOURI & ARKANSAS RWY. CO. | | | PWRC |
| 8 | MAR | *MARTRAC | | | PWRC |
| 9 | MAR | Trafikverket Materialservice Rälsverkstaden | | SE | VKM |
| 10 | MARB | MARIANO BROTHERS, INCORPORATED | | | SCAC4 |
| 11 | MARB | Marathon Bus Service | | US-NJ | TRAFFIC |
| 12 | MARC | Maryland Transit Administration | 7/2005 (not listed) | | NAKINA |
| 13 | MARC | *MARQUETTE CEMENT | | | PWRC |
| 14 | MARC | +MARYLAND TRANSIT ADMINISTRATION | | | PWRC |
| 15 | MARC | MARK IV TRUCKING CO | | | SCAC4 |
| 16 | MARC | Maryland Rail Commuter | | USA | TRAFFIC |
| 17 | MARCIN | Marietta & Cincinnati Railroad | | USA | TRAFFIC |
| 18 | MARD | MARINER DISTRIBUTION INC | | | SCAC4 |
| 19 | MARG | MAR TRANSPORT | | | SCAC4 |
| 20 | MARI | MARELI INTERNATIONAL INC | | | SCAC4 |
| 21 | MARIN | Marin Airporter | | US-CA | TRAFFIC |
| 22 | MARK | MARK TRANS LTD | | | SCAC4 |
| 23 | MARKAB | Zgierskie Przedsiebiorstwo Komunikacyjne "MARKAB" Sp. z o.o. | | PL | TRAFFIC |
| 24 | MARKLIN | Märklin | | X | TRAFFIC |
| 25 | MARL | MARCHAND CONSTRUCTION INC | | | SCAC4 |
| 26 | MARM | Ferrovia Marmifera Privata di Carrara | | I | TRAFFIC |
| 27 | MARN | MARS TRUCKING INC | | | SCAC4 |
| 28 | MARQ | M.O. AIR INTERNATIONAL, INC | | | SCAC1 |
| 29 | MARS | MARS SERVICE COMPANY INC | | | SCAC4 |
| 30 | MART | American Refrigerator Transit Co. | added 4/1960; 4/1961-10/1966; eliminated 10/1969 | | NAKINA |
| 31 | MART | American Refrigerator Transit Co. | 4/61-10/66; eliminated 10/69 | | NRHS |
| 32 | MART | AMERICAN REFRIGERATOR TRANSIT CO. | | | PWRC |
| 33 | MART | Martinair | | | TRAFFIC |
| 34 | MARTA | METROPOLITAN ATLANTA RAPID TRANSIT AUTHORITY | | | PWRC |
| 35 | MARTA | Metropolitation Atlanta Rapid Transit Authority | | USA | TRAFFIC |
| 36 | MARTI | Marti Infra AG | | CH | TRAFFIC |
| 37 | MARTI | Marti Infra AG | | CH | VKM |
| 38 | MARU | CARU CONTAINERS BV | | NL | BIC |
| 39 | MARUB | MARUB SA | | RO | VKM |
| 40 | MARUG | Marugame Community Bus | City: Marugame | J | TRAFFIC |
| 41 | MARW | MAUS, R W TRANSPORT INC | | | SCAC4 |
| 42 | MARW | MARWAY FERTILCHIM SA | | RO | VKM |
| 43 | MARX | MidAmerican Energy Co. | added 10/1998; 7/2000-7/2005 | | NAKINA |
| 44 | MARX | North American Car Corp. | added 7/1955; 10/1957-4/1970; eliminated 7/1973 | | NAKINA |
| 45 | MARX | Mather Stock Car Co. | added 10/1953; 7/1954; eliminated 7/1955 | | NAKINA |
| 46 | MARX | Marland Tank Line Co. (Marland Refining Co.) | 7/1923 | | NAKINA |
| 47 | MARX | Marland Refining Co. | 8/1920 | | NAKINA |
| 48 | MARX | Mather Stock Car Co. | added 10/53; eliminated 7/55 | | NRHS |
| 49 | MARX | North American Car Corp. | added 7/55; 10/57-4/70; eliminated 7/73 | | NRHS |
| 50 | MARX | MidAmerican Energy Co. | added 10/98; 7/2000 | | NRHS |
| 51 | MARX | *MARLAND TANK LINE (MARLAND REFINING CO.) | | | PWRC |
| 52 | MARX | *ARMOUR & CO. (NCCO [MATHER STOCK CAR CO.]) | | | PWRC |
| 53 | MARX | *MISSISSIPPI ALABAMA RAILROAD AUTHORITY | | | PWRC |
| 54 | MARX | +MIDAMERICAN ENERGY COMPANY | | | PWRC |
| 55 | MARX | MidAmerican Energy Corporation | | | RSRV |
| 56 | MARY | MARYVILLE MARITIME INC | | | SCAC4 |
| 57 | MARZ | MARTRAC | | | PWRC |
| 58 | MARZ | MARTRAC | | | RSRV |
| 59 | MARZ | MARTRAC | | | SCAC4 |
