| M Holmes Construction Limited | Construction | 2009 |
| M J Bowen Ltd | Construction | 2016 |
| M J E Servicing Limited | Wholesale & retail | 2002 |
| M J Property Management Services Limited | Real estate | 2019 |
| M Kellas Ltd | Arts & recreation | 2022 |
| M L I Building Products Limited | Wholesale & retail | 1997 |
| M W Fencing and Decking Limited | Construction | 2019 |
| M&B Signature Homes Ltd | Households | 2026 |
| M&F Fire Ltd | Construction | 2025 |
| M&H UK Advisory Ltd | Professional & scientific | 2026 |
| M&J Connections Ltd | Construction | 2015 |
| M-Tech Garage Services Ltd | Wholesale & retail | 2014 |
| M-Tech Investments Limited | Real estate | 2024 |
| M. B. Marine Sales Limited | Wholesale & retail | 1970 |
| M. Giles Electrical Ltd | Construction | 2003 |
| M.A.E. (Southern) Ltd | Manufacturing | 2011 |
| M.A.I.O Inc. Ltd | Administrative & support | 2018 |
| M.A.I.O Ltd | Administrative & support | 2018 |
| M.I.M. Associates Limited | Wholesale & retail | 2018 |
| M.P. Consultants (Surrey) Limited | Administrative & support | 2003 |
| M.S.S. Computer and Business Consultancy Limited | | 1974 |
| MA Cuisine Francaise Ltd | Wholesale & retail | 2015 |
| MA Merchants Ltd | Water & waste | 2025 |
| MA10 Limited | Real estate | 2020 |
| MAA & Kitchen Ltd | Hospitality | 2026 |
| Maarifa Limited | Information & communication | 2018 |
| Mabron Ltd | Health & social care | 2024 |
| MAC Automotive Worthing Limited | Administrative & support | 2018 |
| MAC Autos Worthing Ltd | Wholesale & retail | 2020 |
| Macdonald Homes Investments Ltd | Real estate | 2020 |
| Macdonald Homes South Ltd | Professional & scientific | 2018 |
| Macey Electrical Limited | Construction | 2014 |
| Mach Delivery Ltd | Transport & logistics | 2024 |
| Machinegun Jelly Limited | Arts & recreation | 2003 |
| Mackennal Limited | Construction | 2004 |
| Mackleys Windows Doors and Glazing Ltd | Construction | 2023 |
| Macnab Wallis Associates Ltd. | Administrative & support | 2016 |
| Macrobert Marbleworks Ltd | Construction | 2025 |
| Macware Limited | Information & communication | 2007 |
| MAD Culture Media Ltd | Information & communication | 2024 |
| MAD Heads Digital Ltd | Professional & scientific | 2019 |
| Madam Makery Ltd | Arts & recreation | 2022 |
| Maddisons Limited | Households | 1997 |
| Made 2 Measure Joinery Limited | Construction | 2014 |
| Madray Building Services Limited | Construction | 2015 |
| MAE House Management Limited | Households | 2005 |
| Maenporth Ltd | Information & communication | 2025 |
| Mafi Mushkila Limited | Information & communication | 2002 |
| Magic Happens Limited | Health & social care | 2001 |
| Magic Merlin's Production Ltd | Information & communication | 2019 |
| Magma Media Ltd | Information & communication | 2010 |
| Magnetic Events Ltd | Arts & recreation | 2007 |
| Magnify Marketing Ltd. | Administrative & support | 2005 |
| Mahony Partnership Ltd | Professional & scientific | 2018 |
| Maidment Tanker Services Limited | | 1981 |
| Mainline Renewables Limited | Construction | 2025 |
| Mainline Sewer Services Limited | Water & waste | 2023 |
| Maio Automobili Limited | Wholesale & retail | 2017 |
| Maio Family Investments LLP | | 2025 |
| Maio Group Holdings Limited | Real estate | 2024 |
| Mairon Freight UK Limited | Transport & logistics | 1998 |
| Mairon Motorsport Management Ltd | Professional & scientific | 2024 |
| Maison Juli Ltd | Wholesale & retail | 2019 |
| Majestic Deodorant Productions Limited | Information & communication | 2019 |
| Majestic Marquees Limited | Extraterritorial | 2005 |
| MAK Worthing Ltd | Hospitality | 2020 |
| Make (Good) Trouble C.I.C. | Information & communication | 2018 |
| Makiddies London Limited | Manufacturing | 2021 |
| Making Differently Limited | Wholesale & retail | 2021 |
| Making Food Happen Ltd | Extraterritorial | 2018 |
| Making Sense of IT Psychology Limited | Health & social care | 2018 |
| Mall of Death Ltd | Wholesale & retail | 2025 |
| Malley Carpentry and Building Ltd | Construction | 2024 |
| Mallon Developments Ltd | Real estate | 2019 |
| Mallory Wealth Limited | Financial services | 2021 |
| Mama Still GOT IT! Limited | Other services | 2020 |
| Mamaloveshighstreet Limited | Professional & scientific | 2021 |
| Mamma MIA Steyning Limited | Hospitality | 2018 |
| Mamu LLP | | 2020 |
| Managed Print (UK) Limited | Manufacturing | 2013 |
| Management Matrix Limited | Wholesale & retail | 2000 |
| Mandalay Developments Limited | Real estate | 2007 |
| Mandalay's Child Ltd | Wholesale & retail | 2003 |
| Mander Holdings Limited | Financial services | 2019 |
| Mando Systems Ltd | Information & communication | 2026 |
| Manenti&Co Ltd | Professional & scientific | 2025 |
| Mango Digital Limited | Administrative & support | 2012 |
| Mango N Tango Limited | Wholesale & retail | 2025 |
| Manifold Press Ltd | Manufacturing | 2025 |
| Mannick Holdings Limited | Financial services | 2019 |
| Manny Limited | Information & communication | 2016 |
| Manor Courses Limited | Education | 1977 |
| Manor LEA (Worthing) Limited | Households | 1994 |
| Manormarch Limited | Real estate | 1982 |
| Mansfield Mechanical Ltd | Construction | 2024 |
| Mantaz Fast Food Ltd | Hospitality | 2022 |
| Mantec Computer Services Limited | Information & communication | 1976 |
| Mantrousse Ltd | Arts & recreation | 2015 |
| Manweb Nomineeco Limited | Financial services | 2018 |
| Many Rooms Lettings Limited | Real estate | 2021 |