| Meadowcroft Commissioning Services Ltd. | Professional & scientific | 2017 |
| Mech ENG Consultants Limited | Professional & scientific | 2024 |
| Mech Engineering Services Limited | Construction | 2020 |
| Mech Environmental Ltd | Professional & scientific | 2023 |
| Mechanical BG Ltd | Construction | 2025 |
| Mechanical Wind Turbines Ltd | Professional & scientific | 2024 |
| Mechy Services Ltd | Other services | 2023 |
| Medatraders Ltd | Wholesale & retail | 2023 |
| Media Cultured C.I.C. | Education | 2009 |
| Media Prima Limited | Information & communication | 2014 |
| Medic Hub Limited | Health & social care | 2024 |
| Medical Support Services Ltd | Health & social care | 2018 |
| Medicare Cosmetics Redcar Limited | Health & social care | 2018 |
| Medilinkcoders Ltd | Health & social care | 2025 |
| Medilux Aesthetics Ltd | Health & social care | 2024 |
| Medix Egypt Import & Export Ltd | Manufacturing | 2026 |
| Meehu Property Ltd | Real estate | 2019 |
| Meeks Trading Ltd | Agriculture & fishing | 2019 |
| Meetdcrew Ltd | Administrative & support | 2025 |
| Mefab Limited | Manufacturing | 2022 |
| Mega Books Ltd | Wholesale & retail | 2025 |
| Megabites Express Ltd | Hospitality | 2020 |
| Mehxiv Limited | Administrative & support | 2025 |
| Mejor Tech Solutions Limited | Information & communication | 2025 |
| MEK Wrap Squad Ltd | Wholesale & retail | 2016 |
| Melan Master Construction Ltd | Construction | 2024 |
| Melan Supper Market Ltd | Wholesale & retail | 2025 |
| Melbury Developments (NE) Ltd | Construction | 2025 |
| MEP Builders Limited | Construction | 2023 |
| Merc North East Limited | Wholesale & retail | 2002 |
| Merlin Filtration Limited | Other services | 2018 |
| Merrington Financial Ltd | Financial services | 2019 |
| Mescus Limited | Wholesale & retail | 2025 |
| Mesob 1 Limited | Wholesale & retail | 2022 |
| Metador Holdings Ltd | Financial services | 2025 |
| Metador Limited | Administrative & support | 2019 |
| Metador Property Holdings Limited | Administrative & support | 2019 |
| Methodist House Living Limited | Real estate | 2021 |
| Metro Mini Market (NE) Ltd | Wholesale & retail | 2025 |
| Mewps and Cherry Picker Services Limited | Manufacturing | 2017 |
| MF Developments (N.E) Ltd | Construction | 2012 |
| MF Fabrications Limited | Construction | 2021 |
| MF Inspection Services Ltd | Professional & scientific | 2020 |
| MF SPV Ltd | Real estate | 2025 |
| Mfc.Co.Uk Limited | Extraterritorial | 2000 |
| MFF Renovations Ltd | Construction | 2024 |
| MFS Engineering Ltd | Professional & scientific | 2021 |
| MG Integrity Ltd | Professional & scientific | 2015 |
| MG Motors (M'Bro) Ltd | Wholesale & retail | 2019 |
| MG Power Systems Ltd | Manufacturing | 2024 |
| MG Site Surveying Ltd | Information & communication | 2026 |
| Mggleamclean Ltd | Administrative & support | 2025 |
| Mghz Inspection Ltd | Construction | 2023 |
| Mglascaff Ltd | Construction | 2025 |
| Mgnn Properties Limited | Real estate | 2025 |
| MGP Transport Limited | Transport & logistics | 2019 |
| MGR Commercial Services Limited | Financial services | 2012 |
| MGT Engineering Solutions Ltd | Professional & scientific | 2026 |
| Mgus Properties Ltd | Real estate | 2023 |
| MH Digital Group Ltd | Wholesale & retail | 2018 |
| MH Hameed Logistics Ltd | Transport & logistics | 2025 |
| MH Rescue & Training Ltd | Public administration | 2025 |
| MH Technical Services Limited | Professional & scientific | 2013 |
| MH Wholesales Services Limited | Wholesale & retail | 2025 |
| Mi-Boxlive Limited | Education | 2022 |
| MI2 Group Limited | Construction | 2025 |
| MIA Grace Childrenswear Ltd | Wholesale & retail | 2023 |
| Mial Associates NE (PVT) Limited | Health & social care | 2018 |
| Mian & Co Limited | Professional & scientific | 2007 |
| Michael Atkinson Builder Ltd | Construction | 2025 |
| Michael Conway Associates Limited | Professional & scientific | 2011 |
| Michael J FOX Ltd | Financial services | 2025 |
| Michael O'Byrne Ltd | Real estate | 2022 |
| Michael PEN Limited | Administrative & support | 2013 |
| Michael Poole Limited | Real estate | 2010 |
| Michael Poole Property Consultants Limited | Real estate | 2020 |
| Michael Wood Consulting Teesside Limited | Professional & scientific | 2022 |
| Michelle NE Ltd | Other services | 2024 |
| Michelle Sheekey Transformation Coach Ltd | Other services | 2023 |
| Mick O'Byrne Roofing Ltd | Construction | 2021 |
| Mick Watts Rigging Services Limited | Professional & scientific | 2019 |
| Mick's Bricks Limited | Real estate | 2017 |
| Mickfix Limited | Households | 2025 |
| Mickfix Properties Limited | Real estate | 2026 |
| Mickleby Limited | Financial services | 2025 |
| Mickleton Holdings Limited | Financial services | 2025 |
| Microfran Limited | Other services | 2015 |
| Midas Cladding Services Limited | Construction | 1994 |
| Middlehaven Consultancy Limited | Professional & scientific | 2019 |
| Middlehaven Limited | Real estate | 2020 |
| Middlesborough Murugan Thirukkovil | Other services | 2016 |
| Middlesbrough & District Motor Club Limited | Arts & recreation | 2015 |
| Middlesbrough & Masvingo Alliance CIC | Administrative & support | 2021 |
| Middlesbrough and Stockton Mind Trading Limited | Administrative & support | 2010 |
| Middlesbrough Barbers Limited | Other services | 2025 |
| Middlesbrough Bodywork Shop Ltd | Wholesale & retail | 2026 |
| Middlesbrough Borough Cars Limited | Transport & logistics | 1994 |
| Middlesbrough Brass Castle Golf Club Ltd | Arts & recreation | 2025 |
| Middlesbrough CLT Ltd | Real estate | 2011 |
| Middlesbrough Co-Operative Learning Trust | Education | 2014 |