| Mashariki Creative International Limited | Information & communication | 2023 |
| MAT Mckinley Music Limited | Education | 2002 |
| Matann Metal Fabrication Limited | Manufacturing | 1997 |
| Matform 2000 Limited | Real estate | 1974 |
| Matform Limited | Manufacturing | 1999 |
| Matmos Computer Systems Ltd | Information & communication | 2007 |
| Matridarl Limited | | 1975 |
| Matt Leyton Property Solutions Ltd | Construction | 2025 |
| Matthews Hanton Limited | Professional & scientific | 1989 |
| Matzac Healthcare Ltd | Health & social care | 2022 |
| Maxisearch Limited | Information & communication | 1997 |
| Maxwell's Burger Van Ltd | Hospitality | 2024 |
| Mayfair and Banks Ltd | Construction | 2026 |
| Mayhew Estates (East Grinstead) Limited | Real estate | 2006 |
| Mayhew Holdings Limited | Real estate | 2006 |
| Mayhurst Investments Limited | Real estate | 2007 |
| MB Global Investments Ltd | Real estate | 2019 |
| Mcfarlane Lonie Ltd | Professional & scientific | 2007 |
| Meakins & SON Limited | Wholesale & retail | 1985 |
| Medconsultancy Ltd | Professional & scientific | 2002 |
| Mediclub 2025 LLP | | 2024 |
| Melamy Consulting Ltd | Professional & scientific | 2025 |
| Meldan Reproductions Limited | Manufacturing | 2002 |
| Meldario Ltd | Real estate | 2019 |
| Melford Park Management Company Limited | Real estate | 1999 |
| Memorare House Solutions Ltd | Professional & scientific | 2025 |
| Meralisa Ltd | Wholesale & retail | 2018 |
| Meridian Connect Ltd | Administrative & support | 2024 |
| Merlin Fish Bar Limited | Hospitality | 2016 |
| Metropolis Entertainment Limited | Information & communication | 2007 |
| Mezo Burton Limited | Health & social care | 2013 |
| Mharveycargo Limited | Transport & logistics | 2022 |
| MID Sussex Bathrooms Ltd | Construction | 2005 |
| MID Sussex Forklift Services Limited | Manufacturing | 1995 |
| Mid-Sussex Counselling Centre | Health & social care | 2004 |
| Middleton Medical & Aesthetics Ltd | Health & social care | 2026 |
| Middlewick Training Ltd | Professional & scientific | 2026 |
| Milestones Garage Limited | Wholesale & retail | 1986 |
| Milland Joinery Limited | Manufacturing | 2006 |
| Millen Griffiths Limited | Financial services | 2005 |
| Miller Creative Limited | Administrative & support | 2012 |
| Millmead Properties Limited | Real estate | 1949 |
| Miniguitars Limited | Extraterritorial | 2002 |
| Mirror Make-Up Products Limited | Arts & recreation | 1996 |
| Mission Labels Limited | Manufacturing | 2002 |
| Mitchell's Building Services Limited | | 1983 |
| Mitre Court (Horsham) Management Company Limited | Households | 2000 |
| ML & B Services Limited | Construction | 2023 |
| MNH Consulting Limited | Professional & scientific | 1998 |
| Mobile Telecom International Limited | Professional & scientific | 1999 |
| Modern Ceilings Southern Limited | Construction | 2007 |
| Molica Franklin Ltd | Real estate | 2005 |
| Momentum Asset Management Limited | Real estate | 2004 |
| Moroney Construction Ltd | Professional & scientific | 2016 |
| Morton Universal Limited | Information & communication | 2007 |
| Mosse Associates Limited | Arts & recreation | 2003 |
| Motcombe Grange Limited | Construction | 2002 |
| Motorpilot Ltd | Administrative & support | 2004 |
| Mqueste Communications Limited | Administrative & support | 2003 |
| MR Video (Horsham) Limited | Arts & recreation | 1982 |
| MR. Fixit (Brighton) Limited | Information & communication | 1994 |
| MT Property Consultants Limited | Professional & scientific | 2013 |
| Muktaf Health Care Limited | Health & social care | 2015 |
| Mulberry Court Residents Company (Petworth) Limited | Professional & scientific | 1995 |
| Multi Property Investments Limited | Real estate | 2022 |
| Munro FD Services Limited | Professional & scientific | 2006 |
| Muntham House School | Education | 2004 |
| Murphy Sullivan Associates Limited | Administrative & support | 2003 |
| Mycsp Limited | Financial services | 2011 |
| Mycsp Trustee Company Limited | Professional & scientific | 2011 |
| Mylostone Limited | Real estate | 1998 |
| N & G Clement Limited | Wholesale & retail | 1996 |
| N J Lees Plumbing & Heating Limited | Construction | 2007 |
| N&D Consultants Ltd | Professional & scientific | 2024 |
| N.B.S. Sports & Leisure Limited | Wholesale & retail | 1984 |
| N.E.B. Construction Services Ltd | Construction | 2008 |
| Natalie Read Consultancy Ltd | Professional & scientific | 2025 |
| National Homebuyers Limited | Real estate | 2006 |
| National Hospitality Services (Rickmansworth) Limited | Extraterritorial | 2019 |
| Natures Way Foods Investment Company Limited | Financial services | 2006 |
| Navatmika Ltd | Information & communication | 2026 |
| Neema Amin Consulting Ltd | Professional & scientific | 2025 |
| Netset Management Limited | Administrative & support | 2000 |
| Neutronix Systems UK Limited | Information & communication | 2014 |
| New Barn Garage (Sussex) Limited | Wholesale & retail | 2003 |
| New Place Fencing Limited | Construction | 2002 |
| New Purple Property Limited | Real estate | 2003 |
| New Step Recruitment Ltd | Administrative & support | 2020 |
| New Street Clinic Limited | Health & social care | 1998 |
| Newbridge Property Services Limited | Real estate | 2000 |
| Newlandz Build & Design Developments Limited | Wholesale & retail | 2004 |
| Next Phase Recruitment Limited | Administrative & support | 2003 |
| NGT Waste Services Limited | Water & waste | 2005 |
| NHB Enterprises Limited | Administrative & support | 2004 |
| NHB Investments Ltd | Real estate | 2007 |
| Noah's ARK Childrens Nursery Limited | Education | 2005 |
| Noble Creations Company Limited | Wholesale & retail | 2008 |
| Nordell Group Limited | Extraterritorial | 2004 |
| Normandy Gardens (Horsham) Residents Association Limited | Households | 1974 |
| North Shore Properties Limited | Real estate | 1963 |