| Matthews Heating & Plumbing Limited | Construction | 2016 |
| Mattika Investments Ltd | Real estate | 2022 |
| Mattika Motorhome Hire Ltd | Administrative & support | 2020 |
| Mavel Toyland Limited | Wholesale & retail | 2025 |
| Mavericks Bars (Stockton) Ltd | Hospitality | 2025 |
| MAW Safety Ltd | Construction | 2026 |
| MAX Brothers Ltd | Wholesale & retail | 2018 |
| Maxwell Virtual Solutions Limited | Administrative & support | 2021 |
| MAY William Resource Ltd | Construction | 2018 |
| May's Pizza Limited | Hospitality | 2019 |
| Mayfield Properties (NE) Ltd | Real estate | 2023 |
| Mazegood Limited | Real estate | 2000 |
| Ma’Amj African & Caribbean Cuisine Resturant Ltd | Hospitality | 2025 |
| MB Access Solutions Limited | Construction | 2023 |
| MB Darlington Ltd | Wholesale & retail | 2025 |
| MB Draughting Ltd | Professional & scientific | 2007 |
| MB Electrical and Lettings Limited | Real estate | 2016 |
| MB Masonry and Restoration Ltd | Construction | 2020 |
| MB Services (NE) Limited | Manufacturing | 1989 |
| MBD Consultancy (NE) Ltd | Professional & scientific | 2022 |
| MBDEALS4U Ltd | Wholesale & retail | 2025 |
| MBH Property Limited | Real estate | 2021 |
| MBM Consultancy Ltd | Public administration | 2021 |
| MBS Property Group Ltd | Real estate | 2025 |
| MC Civil & Structural Engineering Services Ltd | Administrative & support | 2023 |
| MC Energy Ltd | Professional & scientific | 2025 |
| MC Laughlin Groundworks Limited | Construction | 2009 |
| MC Property Renovations Limited | Construction | 2018 |
| Mcaveeneyh Limited | Wholesale & retail | 2026 |
| Mcavoy Property Group Limited | Real estate | 2022 |
| MCC Services (North East) Ltd | Construction | 2013 |
| Mccarthy Walker Brickwork Limited | Construction | 2013 |
| Mccaskill Installations Limited | Construction | 2014 |
| Mccrory Silcox Construction Limited | Construction | 2017 |
| Mcdermott Plastering Services (North East) Ltd | Construction | 2019 |
| Mcdonagh Homes Limited | Construction | 2001 |
| Mcdonald NDT Services Limited | Professional & scientific | 2014 |
| Mcje Electrical Ltd | Construction | 2026 |
| Mckays Aspiring Property Ltd | Real estate | 2022 |
| Mckenzie Engineering Services Ltd | Construction | 2026 |
| Mcknight Inspection Ltd | Mining & quarrying | 2024 |
| Mclaren Sports Performance Solutions Ltd | Arts & recreation | 2022 |
| Mclaughlin Inspection Services Ltd | Professional & scientific | 2023 |
| Mcmann Housing Ltd | Real estate | 2025 |
| Mcphee Consultancy Limited | Administrative & support | 2014 |
| MCQ Property Maintenance Ltd | Construction | 2022 |
| MCW Design & Build Ltd | Professional & scientific | 2023 |
| MD Acrylics & Displays Ltd | Wholesale & retail | 2025 |
| MD Allinson Properties Limited | Real estate | 2022 |
| MD Joinery and Building Contractors Limited | Construction | 2019 |
| MD Window Cleaning Ltd | Administrative & support | 2015 |
| Mdavies Properties Ltd | Households | 2023 |
| Mdaviesd Ltd | Health & social care | 2024 |
| MDO Inspections Ltd | Professional & scientific | 2015 |
| Mdsi Construction Ltd | Construction | 2024 |
| Mdsi Group Ltd | Professional & scientific | 2023 |
| Mdsi Property & Development Ltd | Real estate | 2024 |
| MDSI2 Ltd | Real estate | 2023 |
| MDZ Restaurant Ltd | Hospitality | 2019 |
| ME Charlton Ltd | Real estate | 2016 |
| Meadale Limited | Real estate | 2012 |
| Meadowcroft Supported Housing CIC | Real estate | 2025 |
| Meadowfield Gardens Landscaping Limited | Administrative & support | 2003 |
| Meddelivr Technology Limited | Manufacturing | 2025 |
| Medekz Ltd | Education | 2025 |
| Medical Engineering Resource Unit | Manufacturing | 1975 |
| Medicine Health Ltd | Wholesale & retail | 2021 |
| Medieval Snail Limited | Information & communication | 2022 |
| MEE Pro Limited | Wholesale & retail | 2025 |
| Meehu Retail Limited | Wholesale & retail | 2019 |
| Meek Business Consultancy Ltd | Information & communication | 2025 |
| Meepco Limited | Construction | 2024 |
| Meeruworks Ltd | Wholesale & retail | 2024 |
| Megan Limited | Wholesale & retail | 2025 |
| Megan's Skin Club Ltd | Other services | 2023 |
| Meggie Ltd | Wholesale & retail | 2020 |
| Meles Financial Services Limited | Financial services | 2025 |
| Meliora Partners Limited | Real estate | 2021 |
| Melodia Home of Fashion Ltd | Wholesale & retail | 2025 |
| Melodie Wagner Ltd | Arts & recreation | 2022 |
| Meltz of Yarm Ltd | Hospitality | 2025 |
| Memzies Limited | Other services | 2023 |
| MEN Services Ltd | Construction | 2025 |
| Mendal Motors Limited | Wholesale & retail | 2009 |
| Mendeo Limited | Administrative & support | 2024 |
| Mentoris Limited | Professional & scientific | 2015 |
| Mercurial Properties Limited | Real estate | 2012 |
| Mercury Wealth Holdings Ltd | Financial services | 2020 |
| Mercury Wealth Management Limited | Financial services | 2003 |
| Meridian Bridge Ltd | Wholesale & retail | 2026 |
| Meritgrade Limited | Construction | 1976 |
| Mersen UK Holdings Limited | Manufacturing | 1998 |
| Mersen UK Portslade Limited | Manufacturing | 1997 |
| Mersen UK Teesside Limited | Manufacturing | 1966 |
| Mess Charcuterie and Bar Ltd | Hospitality | 2022 |
| MF Engineering Consultant Limited | Professional & scientific | 2026 |
| MF Projects Ltd | Manufacturing | 2012 |
| MG Hospitality and Events Ltd | Hospitality | 2020 |
| MG Motor Group Limited | Wholesale & retail | 2021 |
| MGB Systems Limited | Information & communication | 2012 |