| Metanoia Consultancy Ltd | Administrative & support | 2017 |
| Metis Asset Management Ltd | Financial services | 2017 |
| Metis Wealth Ltd | Financial services | 2012 |
| Metric Tide Ltd | Professional & scientific | 2025 |
| Meyrick (Haywards Heath) Limited | Real estate | 2010 |
| MG Aircraft Engineering Ltd | Manufacturing | 2023 |
| MG Electrical Services (UK) Limited | Construction | 2004 |
| Mhbd Ltd | Professional & scientific | 2023 |
| MHG Consultant Ltd | Real estate | 2018 |
| Michael Bott Consultancy Limited | Health & social care | 1986 |
| Michael Earle Associates Ltd | Construction | 2014 |
| Michelle Sugar Art Limited | Other services | 2007 |
| Michelmersh Brick & Tile Company Limited | Mining & quarrying | 1997 |
| Michelmersh Brick Holdings PLC | Manufacturing | 1997 |
| Michelmersh Brick UK Limited | Manufacturing | 1990 |
| Michiel Gold Limited | Administrative & support | 2015 |
| Mick Jackson and Associates Limited | Information & communication | 2006 |
| Micro ID Technologies Ltd | Manufacturing | 2018 |
| Micro-Id Ltd | Real estate | 2008 |
| MID Sussex Cleaning Ltd | Administrative & support | 2024 |
| MID Sussex Collectibles Ltd | Wholesale & retail | 2025 |
| MID Sussex Construction Ltd | Construction | 2013 |
| MID Sussex Football League Limited | Arts & recreation | 2013 |
| MID Sussex Hearing Care Limited | Wholesale & retail | 2024 |
| MID Sussex Runners Ltd | Administrative & support | 2024 |
| Mid-Sussex Postgraduate Medical Centre Trust Limited | Education | 1975 |
| Middle 8 Digital Limited | Professional & scientific | 2022 |
| Middle Windrushes Ltd | Households | 2022 |
| Midgar Holdings Limited | Financial services | 2019 |
| Midsussexvapourblasting Ltd | Manufacturing | 2019 |
| Mighty Holdings Limited | Financial services | 2021 |
| MIH Associates Ltd | Wholesale & retail | 2024 |
| Mill Green House Limited | Real estate | 2011 |
| Millbanks Property Holdings (MPH) LLP | | 2024 |
| Millcroft & York Lodge Care Homes Limited | Health & social care | 2002 |
| Millhouse Enterprises Limited | Real estate | 2012 |
| Millwood Property Management Limited | Other services | 2026 |
| Millyard GRC Consulting Limited | Administrative & support | 2016 |
| Milne Watson Limited | Manufacturing | 2022 |
| Minchin Limited | Hospitality | 1990 |
| Mineral Supplies International Limited | Other services | 2009 |
| Minerva Workshop Limited | Education | 2019 |
| Miniature Manors Limited | Construction | 2016 |
| Minilabel Limited | Manufacturing | 2006 |
| Mirabilis Holdings Limited | Real estate | 2024 |
| Miraden Holding Limited | Wholesale & retail | 2023 |
| Missing Link Brewing Ltd | Manufacturing | 2017 |
| Mitchell Roofing Services Ltd | Construction | 2024 |
| Mitchinson Macken Ltd | Professional & scientific | 2010 |
| MJ Canton Limited | Hospitality | 2024 |
| MJ Lomas Properties Limited | Real estate | 2017 |
| MJH the Independent Financial Adviser Limited | Financial services | 2001 |
| MJM Invests Limited | Real estate | 2023 |
| MJO Consultancy Limited | Professional & scientific | 2004 |
| MJV Plant Limited | Construction | 2019 |
| MK Design Studio Ltd | Professional & scientific | 2019 |
| MK Steven Property Ltd | Real estate | 2023 |
| MKC Mechanical Services Ltd | Construction | 2024 |
| MKM Property Limited | Real estate | 2026 |
| ML Story Limited | Professional & scientific | 2021 |
| Mldf Holdings Limited | Real estate | 2017 |
| Mlra Ltd | Households | 2011 |
| Mm&L Limited | Administrative & support | 2015 |
| MMA on Point Limited | Information & communication | 2018 |
| MMB Smile Studio Ltd | Health & social care | 2026 |
| MML Finance Holdings Ltd | Professional & scientific | 2023 |
| MMS Homes Limited | Real estate | 2014 |
| Mnst DTV Consulting Limited | Information & communication | 2024 |
| Mobile Commercials Ltd | Transport & logistics | 2003 |
| Mobile VET Sussex Ltd | Professional & scientific | 2025 |
| Mochimoosprints Ltd | Manufacturing | 2024 |
| Moda Industries Limited | Wholesale & retail | 2019 |
| Modern Affluence Limited | Professional & scientific | 2019 |
| Modern Affluence Summit Limited | Professional & scientific | 2019 |
| Mogridge Holdings Ltd | Financial services | 2025 |
| Moher Consulting Limited | Professional & scientific | 2019 |
| Mojo Swoptops Productions Ltd | Information & communication | 2009 |
| Moko J Ltd | Manufacturing | 2025 |
| MON Petit Salon Ltd | Other services | 2024 |
| Monfletch Consulting Limited | Professional & scientific | 2019 |
| Monroe Property Maintenance Ltd | Other services | 2014 |
| Monstrous Thoughts Ltd | Professional & scientific | 2021 |
| Montybay Properties Limited | Real estate | 2025 |
| MOO UN Limited | Information & communication | 2017 |
| Moops Media Limited | Information & communication | 2005 |
| Moore Charles Limited | Professional & scientific | 2016 |
| Moore Fans Limited | Manufacturing | 1975 |
| Moore News Ltd | Information & communication | 2004 |
| Morgan Supply Co Ltd | Wholesale & retail | 2026 |
| Mortice Limited | Administrative & support | 2017 |
| Mortimer and Perkins Ltd | Real estate | 2026 |
| Mosaic MED Ltd | Health & social care | 2022 |
| Mosaic Media 360 Ltd | Information & communication | 2023 |
| Moss Dining Group Limited | Hospitality | 2026 |
| Motivation Limited | Health & social care | 2000 |
| Moto Strada Ltd | Wholesale & retail | 2025 |
| Motor Data Solutions Limited | Information & communication | 2001 |
| Motorcycle Garage Limited | Wholesale & retail | 2020 |
| Mountrichard Limited | Real estate | 2003 |
| Movision Entertainment Limited | Information & communication | 2001 |