| Lucka Construction Ltd | Administrative & support | 2016 |
| Lucy Bendle Physiotherapy Ltd | Other services | 2008 |
| Lucy Simone Limited | Wholesale & retail | 2025 |
| Ludlum Supplies Limited | Transport & logistics | 2009 |
| Lumba Studio Ltd | Professional & scientific | 2019 |
| Lume Sourcing Services Ltd | Professional & scientific | 2017 |
| Lustre Beauty Ltd | Wholesale & retail | 2025 |
| LUX Construction Limited | Construction | 2009 |
| Luxe Studio Beauty Salon Limited | Other services | 2024 |
| Luxe Studio Training Academy Limited | Education | 2024 |
| Luxury Carpets & Flooring Ltd | Wholesale & retail | 2018 |
| LV Cable Solutions Ltd | Construction | 2022 |
| LV Wastewise Ltd | Water & waste | 2026 |
| LVR Bulk Transport Ltd | Transport & logistics | 2020 |
| LVT Design Limited | Professional & scientific | 2019 |
| LW Partners Ltd | Professional & scientific | 2026 |
| Lwfp Investments Ltd | Real estate | 2016 |
| LWJ Rope Tech Ltd | Construction | 2022 |
| Lyall Events and Entertainment Ltd | Administrative & support | 2018 |
| Lynch Collective Ltd | Wholesale & retail | 2024 |
| Lynne Cullen Nutrition Ltd | Health & social care | 2023 |
| Lynsey Davison SWA Ltd | Health & social care | 2021 |
| Lynxlaw Limited | Professional & scientific | 2013 |
| Lyons Four Ltd | Professional & scientific | 2015 |
| Lyons Watson Associates Ltd | Professional & scientific | 2015 |
| Lyons Watson Consultancy Ltd | Professional & scientific | 2020 |
| Lytestream Electrical Engineers & Contractors Limited | Construction | 1993 |
| L’ Alexia Atelier Ltd | Other services | 2023 |
| M & C (Fish and Chips) Ltd | Hospitality | 2024 |
| M & M Darlington Property Limited | Real estate | 2021 |
| M Baxter Marketing Limited | Professional & scientific | 2025 |
| M Cooper Elec Limited | Manufacturing | 2015 |
| M D Emmerson Properties Ltd | Real estate | 2025 |
| M D Hartley's Ltd | Construction | 2013 |
| M Foofstuff Trading and Cleaning Co Ltd | Wholesale & retail | 2025 |
| M H Gorman Electrical Contractors Limited | Construction | 2019 |
| M J English Developments Ltd | Households | 2024 |
| M J Gent Consultancy Limited | Financial services | 2023 |
| M James Technical Services Limited | Construction | 2023 |
| M K Holdings Ltd | Financial services | 2008 |
| M Lawson Ltd | Wholesale & retail | 2024 |
| M M Ullah Ltd | Health & social care | 2022 |
| M Naylor Transport Limited | Transport & logistics | 2017 |
| M New Contracting Ltd | Agriculture & fishing | 2021 |
| M Nicholson Properties Ltd | Real estate | 2026 |
| M R Global Trade Limited | Wholesale & retail | 2025 |
| M S Vernon Limited | Agriculture & fishing | 2014 |
| M Sutton Joinery Ltd | Construction | 2024 |
| M&J Medics Limited | Health & social care | 2016 |
| M&J Property Estates Ltd | Real estate | 2023 |
| M&M Total Property Services Ltd | Construction | 2019 |
| M&T Print and Embroidery Ltd | Manufacturing | 2023 |
| M&T Sportswear Ltd | Wholesale & retail | 2014 |
| M-Mac (Mental Health & Substance Misuse) Ltd. | Health & social care | 2012 |
| M. Dent Farming Limited | Agriculture & fishing | 2015 |
| M.R Clad Ltd | Construction | 2023 |
| M.Rahanu Ltd | Real estate | 2023 |
| M.W Health and Safety Specialist Ltd | Professional & scientific | 2024 |
| M1 Studios Limited | Professional & scientific | 2023 |
| M3 Design Services Limited | Professional & scientific | 1990 |
| M3 MOT Limited | Wholesale & retail | 2025 |
| MAC International Foodservice Limited | Wholesale & retail | 2019 |
| Mach Haul Ltd | Transport & logistics | 2019 |
| Macka & Co Property Ltd | Real estate | 2023 |
| MAD Garage Darlington Ltd | Wholesale & retail | 2025 |
| MAD Parts Darlington Ltd | Wholesale & retail | 2025 |
| MAD Properties North East Ltd | Real estate | 2024 |
| Madame Hair Limited | Other services | 2018 |
| Madarch Ltd | Real estate | 2025 |
| Madbouly Wholesale and Retail Trading Ltd | Wholesale & retail | 2025 |
| Maddams Limited | Wholesale & retail | 2025 |
| Made by Caffeinated Ltd | Information & communication | 2026 |
| Madhavan Healthcare Limited | Health & social care | 2014 |
| Madoodles Doodles Ltd | Manufacturing | 2025 |
| Madtns Delivery Limited | Transport & logistics | 2022 |
| MAE Financial Ltd | Professional & scientific | 2013 |
| Maestro Healthcare Ltd | Health & social care | 2014 |
| MAF Plumbing Heating & GAS Services Limited | Construction | 2016 |
| Magdalen Associates Ltd | Professional & scientific | 2019 |
| Maggie Hart Consultancy Ltd | Education | 2017 |
| Magic and ME Ltd | Health & social care | 2024 |
| Magnet Group Co. Limited | Manufacturing | 2006 |
| Magpie Implantology Ltd | Education | 2026 |
| Maguire TA Limited | Wholesale & retail | 2014 |
| Mahdi & Jannath Enterprise Limited | Wholesale & retail | 2014 |
| Maidendale Limited | Real estate | 2022 |
| Maine Blueprints Ltd | Professional & scientific | 2024 |
| Mainsforth Medical Ltd | Health & social care | 2012 |
| Maitland Agency Limited | Professional & scientific | 2015 |
| Majestc Optima Furniture Ltd | Other services | 2025 |
| Majestic Interiors Limited | | 1987 |
| Maka Logistics Limited | Transport & logistics | 2025 |
| Make IT Mobile Group Ltd | Manufacturing | 2022 |
| Makuch Trans Ltd | Transport & logistics | 2018 |
| Malafolla Ltd | Hospitality | 2025 |
| Malcolm Hepper Framing Limited | Manufacturing | 2013 |
| Malik ALI Promotions Ltd | Arts & recreation | 2022 |
| Malik International Ltd | Wholesale & retail | 2022 |
| Malory Group Ltd | Water & waste | 2026 |
| Malt Hour Ltd | Real estate | 2025 |