| Middlesbrough College Management Services Ltd. | Administrative & support | 1993 |
| Middlesbrough Conservative Club Limited(The) | Hospitality | 1898 |
| Middlesbrough Containers Limited | | 1983 |
| Middlesbrough Cricket Club Limited | Arts & recreation | 2019 |
| Middlesbrough Environment City Trust Limited | Education | 1997 |
| Middlesbrough Estates Ltd | Real estate | 2021 |
| Middlesbrough Fisheries Ltd | Hospitality | 2025 |
| Middlesbrough Football Club Foundation | Education | 1995 |
| Middlesbrough Glass and Glazing Limited | Manufacturing | 1989 |
| Middlesbrough Golf Club Limited | Arts & recreation | 1936 |
| Middlesbrough Hackney Limited | Administrative & support | 2014 |
| Middlesbrough Health and Wellness Ltd | Health & social care | 2025 |
| Middlesbrough Healthcare Limited | Wholesale & retail | 2023 |
| Middlesbrough Judo Ltd | Arts & recreation | 2013 |
| Middlesbrough Lettings Ltd | Real estate | 2023 |
| Middlesbrough Little Theatre Limited(The) | Arts & recreation | 1956 |
| Middlesbrough M S Therapy Centre Limited | Health & social care | 1988 |
| Middlesbrough Mela CIC | Arts & recreation | 2023 |
| Middlesbrough MOT Centre Limited | Wholesale & retail | 2023 |
| Middlesbrough Motors Ltd | Wholesale & retail | 2019 |
| Middlesbrough Music Academy Limited | Education | 2024 |
| Middlesbrough Partners Ltd | Wholesale & retail | 2025 |
| Middlesbrough Port Ltd | Transport & logistics | 2019 |
| Middlesbrough Properties Limited | Real estate | 2019 |
| Middlesbrough Property Investments Ltd | Real estate | 2022 |
| Middlesbrough PUB Group Ltd | Hospitality | 2025 |
| Middlesbrough Rugby Union Football Club Limited | Arts & recreation | 2018 |
| Middlesbrough Storage Ltd | Transport & logistics | 2025 |
| Middlesbrough Sunbed Hire Ltd | Administrative & support | 2020 |
| Middlesbrough Tyre & Auto Centre Limited | Wholesale & retail | 2012 |
| Middlesbrough Voluntary Development Agency | Information & communication | 2002 |
| Middlestone Holdings Limited | Professional & scientific | 2024 |
| Middlesure Limited | Professional & scientific | 2024 |
| Middleton Snails Ltd | Information & communication | 2025 |
| Midkemia Capital Limited | Administrative & support | 2018 |
| Midnight Mirage Productions Ltd | Information & communication | 2023 |
| Miego Ltd | Other services | 2017 |
| Miestilo Projects Limited | Construction | 2026 |
| Mighty Bucks Limited | Construction | 2017 |
| Mihaela Fencing Services Ltd | Professional & scientific | 2025 |
| Mihai Gabi Mitu Logistics Limited | Transport & logistics | 2025 |
| Mihtab Holding for Investments Limited | Construction | 2024 |
| Mike Hind Fitness Limited | Hospitality | 2017 |
| Mikekly Ltd | Construction | 2026 |
| Miketrack Limited | Construction | 2020 |
| Mila Estates Limited | Real estate | 2023 |
| Milano's TS6 Limited | Hospitality | 2024 |
| Mile High Site Services Ltd | Construction | 2026 |
| Mileham Electrical Ltd | Construction | 2024 |
| Milestone Commercials Limited | Real estate | 2011 |
| Miller & Cleaver Ltd | Construction | 2026 |
| Millie Lettings Limited | Construction | 2017 |
| Millington's Electrical Services Limited | Construction | 2026 |
| Milton Grange Engineering Limited | Transport & logistics | 2007 |
| Milton House (Leeds) Freehold Limited | Households | 2025 |
| Mindful in Art Ltd | Education | 2025 |
| Mindset Safety Ltd | Professional & scientific | 2025 |
| Minella Gifts Ltd | Wholesale & retail | 2024 |
| Mingo Properties Limited | Real estate | 2024 |
| Mini Masters Education Ltd | Education | 2025 |
| Minivations North East Ltd | Transport & logistics | 2017 |
| Minster Self-Drive Limited | Administrative & support | 2005 |
| Minster Vehicle Sales LLP | | 2019 |
| Mint Joinery Solutions Ltd | Construction | 2015 |
| Mint Leaf Teesside Ltd | Hospitality | 2024 |
| Mint Mint Nails DH Ltd | Other services | 2024 |
| MIR Gemstones Ltd | Manufacturing | 2019 |
| Miramor Ltd | Other services | 2024 |
| Miss I NE Limited | Other services | 2015 |
| Mission 08 Limited | Arts & recreation | 2024 |
| Mist Lounge Newcastle Ltd | Wholesale & retail | 2019 |
| Mist Snooker Ltd | Arts & recreation | 2025 |
| Mitchell Anton Limited | Wholesale & retail | 2015 |
| Mitchell Wadsworth Ltd | Construction | 2025 |
| Mitia Limited | Professional & scientific | 2016 |
| Mitra Godiva Enterprises Limited | Hospitality | 2024 |
| Mitre Fit Fencing Ltd | Construction | 2024 |
| Mitre-Fit Joinery and Kitchen Installations Ltd | Construction | 2022 |
| Mits Engineering Limited | Professional & scientific | 2023 |
| Mits Group Holdings Limited | Professional & scientific | 2019 |
| Mits Inspection Limited | Professional & scientific | 2023 |
| Mits Property Investments Ltd | Real estate | 2022 |
| Mits Rope Access Limited | Professional & scientific | 2023 |
| MJ Brown Interiors Limited | Construction | 2014 |
| MJ Events (UK) Limited | Other services | 2013 |
| MJ Safety Consultancy Ltd | Professional & scientific | 2025 |
| MJ Telecoms Ltd | Information & communication | 2016 |
| Mjalder Holdings Limited | Financial services | 2022 |
| Mjb&Elb Ltd | Hospitality | 2025 |
| MJC Consultant Services Ltd | Professional & scientific | 2022 |
| MJC NE Ltd | Construction | 2023 |
| Mjdynamic Ltd | Transport & logistics | 2026 |
| MJF Clinical Services Limited | Health & social care | 2022 |
| MJG Services Ltd | Manufacturing | 2025 |
| MJH Properties (NE) Limited | Real estate | 2018 |
| MJH Shopfitting & Joinery Ltd | Construction | 2026 |
| Mjhm Engineering Ltd | Energy | 2019 |
| Mjik Consultancy Services Ltd | Professional & scientific | 2022 |
| MJK Energy Consultants Ltd | Professional & scientific | 2020 |
| Mjlb Developments Ltd | Real estate | 2025 |