| Lynemouth Turkish Barber Ltd | Other services | 2025 |
| Lynnewood LOG Cabins Limited | Real estate | 2025 |
| Lyons Phillips People Management Limited | Administrative & support | 2008 |
| M & C Energy Solutions UK Limited | Construction | 2020 |
| M C Buckland Limited | Real estate | 2002 |
| M Custom Coaching Ltd | Arts & recreation | 2025 |
| M Dungait & Sons Limited | Agriculture & fishing | 1997 |
| M Finn Steelfixing Contractors Limited | Construction | 2010 |
| M Flynn Property Ltd | Real estate | 2024 |
| M Thompson Building Surveyor Ltd | Professional & scientific | 2023 |
| M Weightman Wind Limited | Manufacturing | 2025 |
| M&C Cameron Property Ltd | Real estate | 2021 |
| M. P. L. Investment Ltd | Real estate | 2015 |
| M.J.B Construction Longhirst Ltd | Construction | 2005 |
| M.Scott & Sons Paving Limited | Construction | 2021 |
| Mac-Simec Ltd | Professional & scientific | 2010 |
| Macaulay Farming Ltd | Agriculture & fishing | 2026 |
| Macgregor Build Limited | Construction | 2021 |
| Machining and Fabricating Limited | Manufacturing | 2011 |
| Mackenzie Robson Limited | Professional & scientific | 2010 |
| Made Simply Perfect Events Ltd | Other services | 2025 |
| Made Simply Perfect Limited | Other services | 2016 |
| Maggies Chorus Ltd | Wholesale & retail | 2024 |
| Magnetic Cable Tray Solutions Ltd | Manufacturing | 2025 |
| Magnetic International UK Ltd | Wholesale & retail | 2023 |
| Magpie Ventures Limited | Real estate | 2002 |
| Magyar Renewables Ltd | Professional & scientific | 2018 |
| Maid to Butler Ltd | Households | 2011 |
| Maid to Care Limited | Health & social care | 2015 |
| Mais Sobre Niobio Ltd | Professional & scientific | 2024 |
| Making Changes NE C.I.C. | Education | 2021 |
| Malone Interiors (Northern) Limited | Construction | 2019 |
| Malsiemadhe Ltd | Hospitality | 2026 |
| Manley Property Group Ltd | Real estate | 2025 |
| Manor Fire & Security Solutions Limited | Administrative & support | 2026 |
| Manzil of Morpeth Limited | Hospitality | 2002 |
| Maple Orthopaedics Ltd | Health & social care | 2026 |
| Mapped Law Ltd | Education | 2026 |
| Marabini - Ciro Ltd | Hospitality | 2019 |
| Marcus Child Limited | Arts & recreation | 2020 |
| Marcus Child Online Limited | Arts & recreation | 2024 |
| Marg (N.E.) Ltd | Administrative & support | 2021 |
| MARINAWEST4 Limited | Real estate | 2024 |
| MARINAWEST7 Limited | Real estate | 2024 |
| Marine Mega Store Limited | Wholesale & retail | 2005 |
| Mariners Court (Amble) Limited | Households | 1990 |
| Marius Hodgson Ltd | Financial services | 2020 |
| Mark’S Tech Services Ltd | Construction | 2024 |
| Marley Developments Ltd | Construction | 2021 |
| Marlish Management Company Limited | Real estate | 2002 |
| Marlish Waters Limited | Manufacturing | 2013 |
| Marlo Labs Ltd | Information & communication | 2026 |
| Marshseis Limited | Professional & scientific | 2015 |
| Martin Bourne Limited | Information & communication | 2025 |
| Martin Drake Engineering Limited | Professional & scientific | 1989 |
| Martin Nugent Elite Performance Athlete Development Ltd | Arts & recreation | 2017 |
| Mascot Investments Limited | Financial services | 2025 |
| Masdelivery Ltd | Transport & logistics | 2026 |
| Masons Arms (Amble) Limited | Hospitality | 2016 |
| Masons Electrical Contractors Limited | Construction | 2018 |
| Maughan Engineering Limited | Other services | 1999 |
| Maxcommercial Limited | Administrative & support | 2009 |
| Maxhire Limited | Administrative & support | 1998 |
| Maximize Education Ltd | Education | 2023 |
| MC Propco Ltd | Real estate | 2022 |
| MC Stanley Ltd | Information & communication | 2013 |
| MC-1 International Limited | Wholesale & retail | 2001 |
| Mcbryde & Co Ltd | Professional & scientific | 2019 |
| Mccallum Residential Ltd | Real estate | 2025 |
| Mccoull Property Ltd | Real estate | 2024 |
| Mccutcheon Electrical Contractors Limited | Construction | 2008 |
| Mchugh Holdings Limited | Professional & scientific | 2014 |
| Mckenzie Property Ltd | Real estate | 2022 |
| MDC Automotive Limited | Wholesale & retail | 2019 |
| Meadow Viewers Limited | Households | 2024 |
| Measuretec Ltd | Professional & scientific | 2007 |
| Medatanai Ltd | Real estate | 2020 |
| Medibio Ltd | Professional & scientific | 2014 |
| Medirocs Ltd | Health & social care | 2018 |
| Mediterranean Morpeth Ltd | Hospitality | 2023 |
| Medline Marine Distribution Limited | Wholesale & retail | 2010 |
| Megaptera Ltd | Manufacturing | 2022 |
| Meia Ltd | Other services | 2020 |
| Meia Weddings Ltd | Other services | 2023 |
| Meldon Forge & Farriers Ltd | Other services | 2026 |
| Meldon P Developments Ltd | Construction | 2024 |
| Mercurius Music Ltd | Information & communication | 2021 |
| Merlin Murray Ltd | Arts & recreation | 2021 |
| Mero Management Ltd | Real estate | 2017 |
| Mesh (Strategy) Limited | Professional & scientific | 2017 |
| Metzero Technologies Ltd | Energy | 2023 |
| Mgnjaj Consulting Limited Liability Partnership | | 2007 |
| Michol Ltd | Real estate | 2024 |
| Michol Maintenance Ltd | Real estate | 2025 |
| MID Northumberland Crop Services LLP | | 2007 |
| Middle Moor Management Limited | Households | 2025 |
| Middle Stobswood Farm Limited | Agriculture & fishing | 2018 |
| Mike Platt Audio Ltd | Arts & recreation | 2018 |
| Miller Music and Media Ltd | Information & communication | 2014 |
| Millers Estates Ltd | Real estate | 2022 |