| Mailing Expert Limited | Manufacturing | 2011 |
| Maison Bellis Ltd | Health & social care | 2025 |
| Malling Place (Lewes) Residents Association Limited | Households | 1966 |
| Mandy Chapman Consulting Ltd | Professional & scientific | 2019 |
| Manor House Court Residents Association (Uckfield) Limited | Households | 1988 |
| Manor Twiglets Nursery Limited | Education | 2009 |
| Mansell Mctaggart Uckfield Limited | Real estate | 2003 |
| Mardella Limited | Wholesale & retail | 2020 |
| Maresfield Coaches Ltd | Transport & logistics | 2025 |
| Maresfield Park Management Company Limited | Households | 2009 |
| Maresfield Park Roads Association Ltd | Households | 1965 |
| Margery Properties Limited | Real estate | 1950 |
| Maria Carmen Aesthetics Ltd | Other services | 2022 |
| Maria Magro Designs Ltd | Manufacturing | 2013 |
| Marina Bridal Limited | Wholesale & retail | 2014 |
| Mark Howard Enterprises Limited | Financial services | 2025 |
| Mark’S Deli Ltd | Hospitality | 2025 |
| Marley BOY Ltd | Wholesale & retail | 2023 |
| Marten Land and New Homes Limited | Real estate | 2010 |
| Massey Occupational Health Consulting Limited | Health & social care | 2016 |
| Massgrowth Pro Limited | Other services | 2023 |
| Masterbarberbroadway Ltd | Administrative & support | 2017 |
| Matthew and Stephen Garden Services Ltd | Administrative & support | 2017 |
| Mayco Properties Limited | Real estate | 1973 |
| Mayfield Court Management (Burgess Hill) Limited | Households | 1983 |
| Mccanns (Uckfield) Limited | Other services | 2008 |
| MCD Creative Limited | Arts & recreation | 2017 |
| Mcfarlane Growth Advisory Ltd | Professional & scientific | 2026 |
| Mcfinndon Ltd | Households | 2024 |
| MCI Partners Ltd | Professional & scientific | 2024 |
| MDS Film Locations Management Limited | Information & communication | 2013 |
| Meadow Place Company Limited | Households | 1970 |
| Meadow Views Management Company Limited | Real estate | 2004 |
| Mediadvise Ltd | Administrative & support | 2024 |
| Mendez Property Development Ltd | Construction | 2014 |
| Mentors for Business Leaders Ltd | Real estate | 2015 |
| Mercury Rail and Transportation Ltd | Professional & scientific | 2015 |
| Meryl Bridal Ltd | Wholesale & retail | 2019 |
| MH180 Holdings Limited | Financial services | 2026 |
| MI Expert Limited | Financial services | 2011 |
| Micro Control Instruments Ltd. | Professional & scientific | 2013 |
| MID Sussex Food Partnership CIC | Hospitality | 2026 |
| MID Sussex Land Ltd | Real estate | 2003 |
| Mid-Sussex Homes Limited | Construction | 1985 |
| Mid-Sussex Properties Limited | Construction | 2009 |
| Milabert (UK) Ltd | Wholesale & retail | 2011 |
| Military-Genealogy.Com Limited | Information & communication | 2006 |
| Mill Chase Limited | Real estate | 2019 |
| Mill Drove Flat Management Company Limited | Real estate | 2011 |
| Millbrook Glass Ltd | Manufacturing | 2018 |
| Mindvox Creative Limited | Arts & recreation | 2025 |
| Mizo Drylining Ltd | Construction | 2026 |
| MJA Refurbishments Limited | Construction | 2011 |
| MJD Designer Homes Ltd | Real estate | 2017 |
| MJD Property Rentals Limited | Real estate | 2020 |
| MJS Platinum Services Ltd | Transport & logistics | 2019 |
| MKM Property Solutions Ltd | Real estate | 2025 |
| MLP Advisory Limited | Other services | 2015 |
| Mobile Cleaning Services (UK) Ltd. | Other services | 2012 |
| Mobile Cleaning Services Ltd. | Other services | 2018 |
| Mobile Cleaning Services Worthing Ltd | Other services | 2018 |
| Moka Coffee Ltd | Hospitality | 2026 |
| Molton Developments Limited | Professional & scientific | 2015 |
| Momo Ltd | Hospitality | 2022 |
| Momos 2011 Ltd | Financial services | 2021 |
| Monaxis Holdings Ltd | Financial services | 2025 |
| Moody Sewage Limited | Water & waste | 2008 |
| Moonka HEY Ltd | Information & communication | 2020 |
| Moova Limited | Transport & logistics | 2016 |
| Morgan Kelly Solicitors Limited | Professional & scientific | 2015 |
| Mornington (Wpmj) Ltd | Wholesale & retail | 2025 |
| Morris Refrigeration Limited | Manufacturing | 2004 |
| Morton Waters Communications Ltd | Professional & scientific | 2019 |
| Moses Farm Management Company Limited | Households | 2005 |
| Motorsport Media Group Limited | Extraterritorial | 2025 |
| MPA Financial Planning Ltd | Financial services | 2015 |
| MR Consult Ltd | Wholesale & retail | 2019 |
| Mrmi Ltd | Hospitality | 2018 |
| MSF Partners Limited | Wholesale & retail | 2007 |
| MT Carpentry & Maintenance Services Ltd | Construction | 2023 |
| Mulbury Designs Ltd | Professional & scientific | 2006 |
| Munk Studios Ltd | Professional & scientific | 2012 |
| Murreeservices Ltd | Wholesale & retail | 2022 |
| My Perfect PA Limited | Administrative & support | 2017 |
| Myfonefix Limited | Wholesale & retail | 2018 |
| Nambr Capital Limited | Professional & scientific | 2024 |
| Naths Drainage Solutions Ltd | Construction | 2026 |
| National Froebel Network Ltd | Other services | 2018 |
| Natlam Software Limited | Information & communication | 2014 |
| Natural Playscapes Ltd | Construction | 2019 |
| NCF Properties Limited | Real estate | 2021 |
| NCH Building Limited | Construction | 2022 |
| NDA Collectibles Ltd | Wholesale & retail | 2014 |
| Nemeth-Holding Ltd | Financial services | 2020 |
| NET Cancer Foundation | Education | 2009 |
| NET Capital Limited | Information & communication | 2019 |
| Netchatter Ltd | Information & communication | 2020 |
| Netline Solutions Ltd | Construction | 2026 |
| Network Line Ltd | Wholesale & retail | 2023 |
| New Century Finance Limited | Financial services | 2000 |