| Marsh & Baxter (Truro) Limited | Extraterritorial | 2015 |
| Marsh & Baxter Properties Limited | Construction | 2001 |
| Marsh and Baxter Limited | Financial services | 2008 |
| Marsh Editing Ltd | Arts & recreation | 2014 |
| Martyn Kemp Motor Car Sales Ltd | Wholesale & retail | 2025 |
| Martyn Kemp Motors Limited | Wholesale & retail | 2018 |
| Maryanne Matthews Coaching and Consultancy Ltd | Professional & scientific | 2022 |
| Massamundo Limited | Information & communication | 2014 |
| Master of Bumpers Ltd | Wholesale & retail | 2013 |
| Master Painting Ltd | Construction | 2025 |
| Master Thatchers Sussex Limited | Construction | 2024 |
| MAT Davidson Limited | Information & communication | 2014 |
| Materials Processing Facility Westfield Ltd | Real estate | 2026 |
| Mathew Crawford Architects Limited | Professional & scientific | 2024 |
| Mavick Ltd | Professional & scientific | 2023 |
| MAX Batten Creative Ltd | Information & communication | 2020 |
| MAX Lightfoot Limited | Information & communication | 2021 |
| MAX Select Ltd | Construction | 2021 |
| Maxad Investments Limited | Construction | 1960 |
| Mayo Wynne Baxter LLP | | 2007 |
| Mcbean's Orchids Limited | Real estate | 2014 |
| Mclean Slate Limited | Information & communication | 2017 |
| MDA Trading Limited | Mining & quarrying | 2019 |
| Measurelab Limited | Information & communication | 2013 |
| Medagent International UK Limited | Administrative & support | 2020 |
| Medconfidential Ltd | Other services | 2013 |
| Media Cage Ltd | Information & communication | 2020 |
| Medici Healthcare Ltd. | Health & social care | 2006 |
| Medici Holdings Ltd. | Health & social care | 2006 |
| Medici Properties Limited | Real estate | 2013 |
| Medici Real Estate Ltd. | Health & social care | 2005 |
| Medieval Lewes Ltd | Real estate | 2017 |
| Medintel Ltd | Professional & scientific | 2022 |
| MEL 56 Real Estate Ltd | Households | 2012 |
| MEL Goddard Consultancy Ltd | Professional & scientific | 2014 |
| Melis 50 Real Estate Ltd | Households | 2012 |
| Mellor + France Limited | Hospitality | 2019 |
| Melsetter Solutions Limited | Information & communication | 2000 |
| Mendfences Ltd | Professional & scientific | 2026 |
| Mercury Media & Events Limited | Information & communication | 2016 |
| Meridian House Holdings Limited | Construction | 2025 |
| Merrydown Estate Management Limited | Real estate | 2010 |
| Merston Community Solar C.I.C. | Energy | 2023 |
| Merston Renewable Energy CIC | Energy | 2015 |
| Metal Roof Ltd | Construction | 2007 |
| Metal Stewardship Limited | Professional & scientific | 2018 |
| Meticulous IT Limited | Information & communication | 2012 |
| MH Fitness Management Limited | Manufacturing | 2008 |
| MHL Build Limited | Construction | 2010 |
| Michael Bennett Consulting Limited | Professional & scientific | 2016 |
| Michael Brooks Ltd | Arts & recreation | 2013 |
| Michael Sinclair Counselling Limited | Administrative & support | 2007 |
| Michaela Kullack Limited | Health & social care | 2003 |
| Mick Ronson Foundation | Other services | 2016 |
| Micro Flats (Sussex) 1 Ltd | Real estate | 2024 |
| Micro Flats (Sussex) 2 Ltd | Real estate | 2024 |
| Micro Flats (Sussex) 3 Ltd | Real estate | 2024 |
| Micro Flats (Sussex) Ltd | Real estate | 2024 |
| MID Sussex Anaesthesia LLP | | 2024 |
| MID Sussex Marlins Swimming Club Limited | Arts & recreation | 2012 |
| Middle Farm Limited | Professional & scientific | 2002 |
| Middle Farm Shop Limited | Extraterritorial | 2008 |
| Middleton Laine Holdings Limited | Professional & scientific | 2013 |
| Midmer Limited | Professional & scientific | 2016 |
| Mikelis Collective Ltd | Professional & scientific | 2024 |
| Mile One Limited | Professional & scientific | 2007 |
| Miles Buckley Show Training Limited | Agriculture & fishing | 2019 |
| Miles Property Management (South East) Limited | Real estate | 2000 |
| Millies and Co Limited | Wholesale & retail | 2013 |
| Milligan Media Limited | Professional & scientific | 2023 |
| Millington Consultancy Services Limited | Hospitality | 2015 |
| Mills Consultants Limited | Professional & scientific | 2006 |
| Mills Trail Running Limited Partnership | | 2019 |
| Millwright Sussex Limited | Households | 2003 |
| Milterbest Signs and Screen Printing Ltd | Professional & scientific | 2009 |
| Mimemento Ltd | Manufacturing | 2025 |
| Mindout Lgb&T Mental Health Project | Health & social care | 2010 |
| Minor Props Limited | Professional & scientific | 2022 |
| Miramar Investment Solutions Limited | Professional & scientific | 2011 |
| Mire & Mettle Ltd | Manufacturing | 2025 |
| Mirror and Mountain Ltd | Information & communication | 2024 |
| Mission Drive Marketing Ltd. | Arts & recreation | 2017 |
| MIT Live Limited | Arts & recreation | 2022 |
| Mitchell Environmental Limited | Professional & scientific | 2020 |
| Mitchell Grounds & Fencing Ltd | Administrative & support | 2021 |
| Mixed in Sheffield Projects Ltd | Arts & recreation | 2013 |
| MJB Architecture Limited | Professional & scientific | 2007 |
| MJP Roofing Group Limited | Construction | 2022 |
| Mobile Shelving UK Limited | Wholesale & retail | 2007 |
| Mode IV Ltd | Arts & recreation | 2020 |
| Modern Hardcore Ltd | Wholesale & retail | 2022 |
| Moisma Limited | Real estate | 2019 |
| Mojo Creations Ltd | Wholesale & retail | 2016 |
| Mojo Haka Property Ltd | Real estate | 2023 |
| Mojo Television Limited | Arts & recreation | 2017 |
| Monkey JR Ltd | Wholesale & retail | 2024 |
| Monkfish Music Ltd | Arts & recreation | 2021 |
| Mont Blanc Adventures Limited | Administrative & support | 2011 |
| Montelona Ltd | Administrative & support | 2025 |
| Moon KIM Theatre Company Ltd | Arts & recreation | 2024 |