| Mission Heating and GAS Limited | Professional & scientific | 2021 |
| MJR1 Limited | Construction | 2019 |
| MJS Security Services Limited | Administrative & support | 2025 |
| MK Transport and Training Solutions Ltd | Professional & scientific | 2026 |
| MMC Financial Limited | Financial services | 2015 |
| Mobius Fitness Ltd | Wholesale & retail | 2023 |
| Modern Classic Cars Limited | Wholesale & retail | 2011 |
| Moderniseconstruct Limited | Construction | 2023 |
| Molecular Fitness Ltd | Arts & recreation | 2023 |
| Monroe Beauty Training Academy Ltd | Education | 2020 |
| Montgomery ST Ltd | Households | 2018 |
| Moret Network Ltd | Information & communication | 2015 |
| Morrow Gardening Services Ltd | Administrative & support | 2022 |
| Moss Engineering Limited | Construction | 2016 |
| Mowlud Mashud Ltd | Hospitality | 2025 |
| MPM Durham Limited | Manufacturing | 1970 |
| MPS Irwin Ltd | Hospitality | 2018 |
| MR Log!Stic Ltd | Transport & logistics | 2023 |
| MR MIX HQ Ltd | Hospitality | 2026 |
| MR Oven Lover Ltd | Administrative & support | 2020 |
| Mreed Renewables Ltd | Other services | 2023 |
| Mrrenewables Ltd | Construction | 2026 |
| Mrrs Mikey Rowlands Rigging Solutions Ltd | Information & communication | 2024 |
| MS Inspection Services Ltd | Construction | 2024 |
| MTK North East Limited | Administrative & support | 2019 |
| MTS Developments (North East) Limited | Construction | 2016 |
| MUNGO1 Ltd | Transport & logistics | 2026 |
| Murray and Lamb Properties Limited | Real estate | 2017 |
| Murray Psychology Limited | Education | 2019 |
| Mychoice Homecare Limited | Health & social care | 2008 |
| Mysticfire Limited | Administrative & support | 2016 |
| N.B.Transport Limited | Transport & logistics | 1948 |
| N.C Food Limited | Hospitality | 2023 |
| N.E. Green Waste Recycling Ltd | Water & waste | 2011 |
| Nats Arbitrage Ltd | Wholesale & retail | 2026 |
| NDW Howd Technical Services Limited | Professional & scientific | 2020 |
| NE Pacific Ltd | Wholesale & retail | 2024 |
| NE Payroll Ltd | Information & communication | 2015 |
| Nearby Durham Taxis Ltd | Transport & logistics | 2012 |
| Nebula Fire and Security Ltd | Administrative & support | 2025 |
| Neil Banerjee and Associates Limited | Health & social care | 2009 |
| Neil Lowdon Limited | Manufacturing | 2015 |
| Network Ventures (UK) Ltd | Professional & scientific | 2011 |
| Neurodivergent Wellbeing and Family Support CIC | Education | 2026 |
| Neurorehab North East Ltd | Health & social care | 2026 |
| Neurotrix Solutions Ltd | Wholesale & retail | 2025 |
| Newcastle Social Ltd | Professional & scientific | 2024 |
| Newmarket Street PUB Company Ltd | Hospitality | 2025 |
| Newton and LEE Automotive Ltd | Wholesale & retail | 2016 |
| Nexavion Ltd | Wholesale & retail | 2026 |
| Next GEN Eco Group Limited | Construction | 2022 |
| Next GEN Eco Solutions Ltd | Administrative & support | 2016 |
| Nguyen Nail Limited | Other services | 2024 |
| Nicholson Heating and Plumbing Ltd | Construction | 2024 |
| Nickynoo Quirky Mobile Bars Ltd | Hospitality | 2023 |
| Nigestan Ltd | Administrative & support | 2024 |
| NJ Brothers Properties Ltd | Real estate | 2025 |
| NLD Fleet Support Ltd | Wholesale & retail | 2025 |
| NLD Tyres Ltd | Wholesale & retail | 2025 |
| NMC Sports Ltd | Administrative & support | 2021 |
| NO 1 Woodfuel Limited | Manufacturing | 2008 |
| NO1 Bathrooms & Cladding Limited | Construction | 2022 |
| Nobas Limited | Health & social care | 2015 |
| Noble's Bingo DH8 Limited | Arts & recreation | 2025 |
| Nord Consultancy Services Ltd | Professional & scientific | 2026 |
| Normec Dets Limited | Professional & scientific | 1999 |
| North Durham Living Ltd | Real estate | 2025 |
| North Durham Safety Limited | Professional & scientific | 2016 |
| North East Decorating Contractors Ltd | Construction | 2024 |
| North East Ground Maintenance Ltd | Administrative & support | 2022 |
| North East Home Staging Limited | Real estate | 2021 |
| North East Power Ltd | Construction | 2023 |
| North East Property Group Ltd | Real estate | 2018 |
| North Road (Boldon) Developments Ltd | Construction | 2018 |
| North West Durham Hairdressing Company Limited | Other services | 2012 |
| Northern Delta Developments Limited | Construction | 2019 |
| Northern Delta Plant Hire Limited | Administrative & support | 2018 |
| Northern Heating Services Ltd | Construction | 2019 |
| Northern Lotus Ltd | Financial services | 2026 |
| Northern NDT Services Ltd | Professional & scientific | 2021 |
| Northfarthing Press Limited | Other services | 2021 |
| Northgen Ltd | Construction | 2024 |
| Northstead Property Group Ltd | Real estate | 2026 |
| Northumberland Rugby Football Union Limited | Arts & recreation | 2011 |
| Northumbria Letting Ltd | Households | 2021 |
| Northwood Edinburgh Limited | Real estate | 2003 |
| Northwood Facilities Ltd | Administrative & support | 2020 |
| Novocastrian Rings Limited | Manufacturing | 2003 |
| NTC Truck and Trailer Services Limited | Wholesale & retail | 2016 |
| Nthice Eron Ltd | Wholesale & retail | 2025 |
| Oak and Iron Property Lettings Ltd | Real estate | 2024 |
| Oakheart Hospitality Ltd | Hospitality | 2026 |
| Oakwood Capital Management Ltd | Real estate | 2021 |
| Oath of Allegiance Ltd | Wholesale & retail | 2024 |
| Obsidian Menswear Ltd | Wholesale & retail | 2026 |
| OD Telecoms Limited | Information & communication | 2025 |
| OHR Business Services Ltd | Professional & scientific | 2020 |
| Ojeautos Limited | Wholesale & retail | 2025 |
| Olddsetum Ltd | Wholesale & retail | 2026 |
| Oldpark Consulting Ltd | Professional & scientific | 1998 |