| Marsh Engineering Services Ltd | Manufacturing | 2024 |
| Martek Electrical South Ltd | Construction | 2020 |
| Martin Penn Independent Driver Hire Ltd | Transport & logistics | 2013 |
| Martino Professional Accountants Ltd | Other services | 2023 |
| Martino Resources Ltd | Agriculture & fishing | 2023 |
| Masharo Ltd | Information & communication | 2013 |
| Mason Heating Plumbing Solar Ltd | Construction | 2011 |
| Matrons Cottages Management Limited | Households | 2016 |
| Matt Jacobs Holdings Ltd | Financial services | 2017 |
| Matthews Clearance and Couriers Ltd | Transport & logistics | 2025 |
| Maverick Hair Studio Limited | Other services | 2008 |
| Maximus Enterprises Limited | Arts & recreation | 2003 |
| Maxtoro Lettings Ltd | Real estate | 2024 |
| Maxtoro Limited | Financial services | 2020 |
| Maycliff Limited | Real estate | 2001 |
| Mayhurst Capital Limited | Professional & scientific | 2015 |
| Mayhurst Developments Limited | Professional & scientific | 2015 |
| Mayleven Ltd | Financial services | 2026 |
| Mayleven Properties Ltd | Financial services | 2026 |
| Mayleven Strand Ventures Ltd | Financial services | 2026 |
| Mazur Auto Service Ltd | Wholesale & retail | 2021 |
| MB Drywall Ltd | Construction | 2008 |
| MC Interiors Ltd | Construction | 2023 |
| MC Kittyz Ltd | Agriculture & fishing | 2022 |
| Mcarthur Black Limited | Real estate | 2015 |
| Mcarthur White Ltd | Real estate | 2022 |
| Mccartain Golf Services Limited | Arts & recreation | 2013 |
| MCG Constraction Ltd | Construction | 2019 |
| Mckechnie Pullan Limited | Professional & scientific | 2003 |
| Mcmillan Consultants Ltd | Information & communication | 2015 |
| MDK Management Company Ltd | Real estate | 2024 |
| MDK Vehicle Repairs Ltd | Wholesale & retail | 2020 |
| Mead Property Estate Ltd | Real estate | 2026 |
| Meadowtree Ltd | Professional & scientific | 2022 |
| Meat in the Square Limited | Wholesale & retail | 2014 |
| Mecenate Ltd | Wholesale & retail | 2019 |
| Medi Procurement (WS) Limited | Administrative & support | 2012 |
| Mediwin Ppip Ltd. | Wholesale & retail | 2001 |
| Medolah Ltd | Health & social care | 2023 |
| Megavent Group Limited | Professional & scientific | 2021 |
| Melius Asset Finance Solutions Ltd | Professional & scientific | 2023 |
| Meowve - South Downs Ltd | Professional & scientific | 2026 |
| Meridian Medical Limited | Manufacturing | 1991 |
| Mescrs Ltd | Financial services | 2025 |
| Metron Records Limited | Information & communication | 2022 |
| Meynell Limited | Real estate | 2010 |
| MHC Arundel Ltd | Hospitality | 2011 |
| MIA Alexander Ltd | Extraterritorial | 2018 |
| Michael Wood Management Consultancy Ltd | Professional & scientific | 2015 |
| Michelle Kendrick Ltd | Professional & scientific | 2024 |
| Micky Flanagan Entertainment Limited | Arts & recreation | 2010 |
| Micron Semiconductor Holdings Limited | Financial services | 2022 |
| Micron UK Limited | Financial services | 2022 |
| MID Sussex Holdings Ltd | Financial services | 2022 |
| Middleton on Sea Estate Association Limited | Households | 2026 |
| Mike Collins Memorial Ltd | Health & social care | 2024 |
| Mil-Hom Limited | Administrative & support | 2004 |
| Milkwood Film Productions Limited | Information & communication | 2011 |
| Milkwood Productions LLP | | 2008 |
| Mill Wood Finance Limited | Financial services | 1998 |
| Millfield Overstrand Limited | Households | 1985 |
| Milo Motion Control Limited | Information & communication | 2013 |
| Miracle Move Ltd | Real estate | 2026 |
| Miravista Limited | Administrative & support | 1999 |
| Miravista Travel Limited | Information & communication | 2013 |
| Mission Fifty Seven Limited | Information & communication | 2024 |
| MJI Hospitality Ltd | Hospitality | 2025 |
| MJK Manufacturing Ltd | Manufacturing | 2021 |
| MJT Automotive Ltd | Wholesale & retail | 2023 |
| MJT Engineering Services Ltd | Wholesale & retail | 2025 |
| MK Blinds & Shutters Ltd | Other services | 2019 |
| MK Manufacturing (Ltd) Limited | Manufacturing | 2021 |
| ML Electrical Services (Sussex) Ltd | Construction | 2017 |
| MLD Event Group Ltd | Professional & scientific | 2021 |
| Mlmcs Ltd | Information & communication | 2012 |
| Mnemonic Trading Limited | Financial services | 2012 |
| Mobile Insight Ltd | Professional & scientific | 2011 |
| Modeaim Limited | Real estate | 1989 |
| Moderno (Brighton) Management Company Limited | Households | 2013 |
| Moka Linked Ltd | Wholesale & retail | 2022 |
| Mokrina International Ltd | Administrative & support | 2025 |
| Molica Franklin (Littlehampton) Limited | Real estate | 2014 |
| Molly Care Ltd | Health & social care | 2020 |
| Monkey Meditation Ltd. | Health & social care | 2015 |
| Monster Music Enterprises Ltd | Information & communication | 2018 |
| Montague Court Residents Association Limited | Households | 1995 |
| Montbain Limited | Financial services | 2018 |
| Morehouse Limited | Real estate | 2021 |
| Morgan Henley Investments Limited | Real estate | 2025 |
| Morgan Henley Properties Limited | Real estate | 2019 |
| Moro & Co Global Ltd | Wholesale & retail | 2026 |
| MOS Enterprises Limited | Construction | 2024 |
| Moto FX Limited | Professional & scientific | 2014 |
| Motorglass Autoglazing Limited | Wholesale & retail | 2017 |
| MOVE2LEARN Ltd | Education | 2026 |
| Moviemania Limited | Extraterritorial | 2011 |
| Moving Power Ltd | Construction | 2021 |
| MP Costcutter UK Limited | Wholesale & retail | 2006 |
| MPG Group Sussex Limited | Extraterritorial | 2015 |
| MPP Consulting Ltd | Professional & scientific | 2018 |