| M&M Future Limited | Real estate | 2024 |
| M&M Handymen Limited | Construction | 2025 |
| MAC Aero Limited | Manufacturing | 2025 |
| Made Outdoors Ltd | Wholesale & retail | 2023 |
| Madean Healthcare Ltd | Health & social care | 2023 |
| Madlautom Limited | Construction | 2015 |
| MAG Signature Sound Ltd | Wholesale & retail | 2024 |
| Mahalaxmi Limited | Wholesale & retail | 2024 |
| MAK Plumbing and Heating Installations Ltd | Construction | 2023 |
| Manna Bites Ltd | Wholesale & retail | 2024 |
| Manor Road Commercial Ltd | Real estate | 2026 |
| Manse Group Ltd | Households | 2023 |
| Mansfield Property Ltd | Real estate | 2019 |
| Marc Ryzer Ltd | Information & communication | 2017 |
| Marine Air Products Limited | Wholesale & retail | 2019 |
| Marine Point Residents Association Limited | Households | 1969 |
| Marius Holdings Ltd | Financial services | 2019 |
| Mark Thomas Trimming Ltd | Manufacturing | 2021 |
| MARK1TECH Ltd | Information & communication | 2017 |
| Market Builder Limited | Real estate | 2004 |
| Marsa & Mariana Limited | Real estate | 2024 |
| Marsh Labels Limited | Manufacturing | 1983 |
| Marshallsay Ford Ltd | Real estate | 2017 |
| Martin NYE (Sussex) Limited | Professional & scientific | 2014 |
| Masau Consulting Limited | Professional & scientific | 2020 |
| Mash OPS Limited | Hospitality | 2026 |
| Mason Progressive Cycles Ltd | Manufacturing | 2014 |
| Masters Building & Demolition Services Ltd | Professional & scientific | 2026 |
| MAX Milo Ltd | Real estate | 2023 |
| May's Chips Limited | Hospitality | 2011 |
| Mcdonalds Properties Limited | Real estate | 2026 |
| MCE Investments Ltd | Real estate | 2013 |
| Mckeown Properties Limited | Hospitality | 2000 |
| Mead Engineering Group Ltd | Manufacturing | 2010 |
| Mead Engineering Services Limited | Manufacturing | 2004 |
| Mead Structures Ltd | Professional & scientific | 2010 |
| Meadow Crescent 1965 Limited | Extraterritorial | 2014 |
| Mearhill Limited | Households | 2003 |
| Media Brighton Ltd | Information & communication | 2016 |
| Medialink Solutions Ltd | Manufacturing | 2018 |
| Meeple Consulting Ltd | Professional & scientific | 2021 |
| Menzies Smith Safety Ltd | Professional & scientific | 2021 |
| Mermaid Care Limited | Health & social care | 2016 |
| Metrocity Consultants Limited | Professional & scientific | 1997 |
| Meze Meze (Lancing) Ltd | Hospitality | 2012 |
| Micrograce Limited | Information & communication | 1999 |
| Milford Court (Lancing) Limited | Households | 2007 |
| Millenium Therapy UK Limited | Wholesale & retail | 2016 |
| Miller & Rockliffe Ltd | Wholesale & retail | 2023 |
| Millwood House Consultants Ltd | Professional & scientific | 2010 |
| Milyma Ltd | Information & communication | 2020 |
| Mindhop Ltd | Administrative & support | 2018 |
| Minding the Soul Limited | Professional & scientific | 2012 |
| Mint Homes Limited | Real estate | 1966 |
| Miranova Ltd | Real estate | 2024 |
| Missa Italian Ltd | Hospitality | 2025 |
| Mitchell and Co Associates Limited | Professional & scientific | 2019 |
| Mixtuur UK Limited | Wholesale & retail | 2020 |
| MJ & Sons Enterprises Holdings Limited | Wholesale & retail | 2022 |
| MJ Lean Consultancy Ltd | Professional & scientific | 2018 |
| Mj&Sons Estates Limited | Construction | 2023 |
| Mjeb Consultancy Services Ltd | Professional & scientific | 2019 |
| MJS Estimating Services Ltd | Construction | 2024 |
| MKS PM Services Ltd | Professional & scientific | 2018 |
| ML Contracts (Kent) Ltd | Construction | 2021 |
| ML Services (Sussex) Ltd | Information & communication | 2022 |
| Mmm-Media Ltd | Information & communication | 2011 |
| Moffatt Design Limited | Professional & scientific | 2017 |
| Mojo Property Group Ltd | Real estate | 2025 |
| Monroe and Chase Limited | Administrative & support | 2009 |
| Monsoon Indian Dining Limited | Hospitality | 2014 |
| Montbond LLP | | 2016 |
| More Than Clearance Limited | Transport & logistics | 2011 |
| More Than Vans Limited | Wholesale & retail | 2024 |
| Morgan Lloyd Properties Ltd | Real estate | 2021 |
| Morris & Turner Investments Limited | Wholesale & retail | 1992 |
| Motel Navigation Limited | Transport & logistics | 2012 |
| Moto-Mate.Ai Ltd | Information & communication | 2025 |
| Motograph Limited | Professional & scientific | 2023 |
| Motorsport Technical Services Ltd | Wholesale & retail | 2013 |
| Moutzousports Ltd | Wholesale & retail | 2020 |
| MP GAS Services Limited | Construction | 2024 |
| MPS Electrics Limited | Construction | 2018 |
| MR Simon Cohen Limited | Professional & scientific | 2013 |
| MR Start Ltd | Wholesale & retail | 2011 |
| MRL Roofing Ltd | Construction | 2021 |
| MSC Properties Limited | Real estate | 2016 |
| MST Retail Ltd | Wholesale & retail | 2024 |
| Muhammad Junaid Ltd | Wholesale & retail | 2025 |
| Mukutenda Ltd | Health & social care | 2022 |
| Mulakat Indian Dining Limited | Hospitality | 2015 |
| Murrin Aviation Solutions Ltd | Administrative & support | 2025 |
| Muse Dance Studios Ltd | Education | 2026 |
| Must Lets Ltd | Real estate | 2023 |
| Mwaysfood Ltd | Administrative & support | 2026 |
| My Recruitment Company Ltd | Professional & scientific | 2020 |
| Nali Sussex Ltd | Wholesale & retail | 2025 |
| Narx Limited | Information & communication | 2011 |
| Nattmedia Ltd | Information & communication | 2023 |
| Nattrass Holdings Ltd | Financial services | 2025 |