| Marilyn Stafford Photography Ltd | Professional & scientific | 2022 |
| Marine and Pneumatics Limited | Wholesale & retail | 2013 |
| Marine Electrician Ltd | Construction | 2026 |
| Marione Delice Limited | Hospitality | 2025 |
| Mark Oakden Surveying Services Ltd | Professional & scientific | 2012 |
| Marketplace-Brighton Ltd | Wholesale & retail | 2008 |
| Martial Arts Gear Store Limited | Wholesale & retail | 2017 |
| Martin & Co Consultancy Services Limited | Professional & scientific | 2022 |
| Martin & Co Probate Services Limited | Professional & scientific | 2021 |
| Martin Petrie Personal Training Limited | Arts & recreation | 2015 |
| Martin-Wells Distillery Ltd. | Manufacturing | 2016 |
| Mary Gold Grows Community Interest Company | Education | 2019 |
| Maslen Consulting Ltd | Information & communication | 2022 |
| Maspad Properties Limited | Real estate | 2023 |
| Master Blaster (South East) Ltd | Administrative & support | 2023 |
| Maswood Limited | Real estate | 2021 |
| Mathemagenic Ltd | Information & communication | 2024 |
| Matrix Platinum Ltd | Information & communication | 1999 |
| Matthew Hitchcock Ltd | Wholesale & retail | 2022 |
| Maxemize Developments Brighton Ltd | Real estate | 2018 |
| Mayco Consulting Limited | Professional & scientific | 2021 |
| Mayco Living Limited | Real estate | 2021 |
| Mazmik Ltd | Wholesale & retail | 2010 |
| Mccarney Consulting Limited | Information & communication | 2014 |
| MCD Electrical Limited | Construction | 2016 |
| ME ME Boutique Ltd | Wholesale & retail | 2009 |
| Meat Wholesale Ltd | Wholesale & retail | 2021 |
| MED Laser Ltd | Other services | 2023 |
| Mental Health United CIC | Arts & recreation | 2024 |
| Mescart Systems Ltd | Information & communication | 2025 |
| Meyle UK Limited | Wholesale & retail | 2009 |
| MHI Networks Ltd | Information & communication | 2019 |
| Michael Levy Limited | Wholesale & retail | 2002 |
| Michael Spisto Ltd | Construction | 2015 |
| Michelle Flynn Coaching Limited | Health & social care | 2018 |
| Microcement Surface Studio Limited | Construction | 2026 |
| Microcret Limited | Construction | 2025 |
| Microsolve Electronics Recruitment Limited | Administrative & support | 2018 |
| Microsolve Technology Recruitment Ltd | Professional & scientific | 2025 |
| MID Removals Limited | Transport & logistics | 2024 |
| MID Sussex Osteopaths Limited | Health & social care | 2023 |
| MID Sussex Scaffolding Limited | Construction | 1991 |
| Mid-Sussex Roofing Limited | Construction | 1982 |
| Midas Mortgage Centre Ltd | Financial services | 2014 |
| Middle Road OFF Licence Ltd | Wholesale & retail | 2026 |
| Midland Telecommunications Management Limited | Information & communication | 1998 |
| Mifune Ltd | Information & communication | 2013 |
| Miint Ltd | Wholesale & retail | 2026 |
| Mike Lebihan Studio Ltd | Professional & scientific | 2017 |
| Millington RTM Company Limited | Households | 2005 |
| Milner Road (Brighton) Limited | Hospitality | 2003 |
| Mind Gains Limited | Education | 2019 |
| Minimal Kids Limited | Information & communication | 2015 |
| Mjpjd Limited | Real estate | 2025 |
| MJR Building & Construction Ltd | Construction | 2014 |
| MKS TCG Limited | Wholesale & retail | 2025 |
| MLM SAS Limited | Wholesale & retail | 2026 |
| Modern Media Communications Limited | Information & communication | 1997 |
| Montys Roofing and Building Ltd | Construction | 2023 |
| More Than Words Tuition Ltd | Education | 2021 |
| Moshi Moshi Coaching Ltd | Professional & scientific | 2024 |
| Moss Communications Ltd | Professional & scientific | 2024 |
| Mowmoo Limited | Professional & scientific | 2023 |
| MR Bongo Publishing Ltd | Information & communication | 2014 |
| MR Plumber Ltd | Construction | 2019 |
| MR. Bongo Worldwide Limited | Wholesale & retail | 1995 |
| Mr.Masey Ltd | Wholesale & retail | 2022 |
| Msg-Vox Ltd | Information & communication | 2023 |
| MT 94 Company Ltd | Other services | 2022 |
| My Wellwise Limited | Information & communication | 2025 |
| Mygymuk Ltd | Arts & recreation | 2015 |
| Mywick Spaces Limited | Real estate | 2026 |
| N E Property Maintenance (Sussex) Ltd | Construction | 2021 |
| Nasrin MO Limited | Health & social care | 2023 |
| Natalie Burtenshaw Ltd | Wholesale & retail | 2022 |
| Natalie Wilson Ltd | Professional & scientific | 2025 |
| Nataliya Blanchard Ltd | Administrative & support | 2006 |
| National Federation of Subpostmasters | Other services | 2015 |
| Natural Lawn Ltd | Administrative & support | 2021 |
| Nb:Architecture Ltd | Professional & scientific | 2011 |
| NC Interiors Limited | Arts & recreation | 2010 |
| Neighbourhood Store Ltd | Wholesale & retail | 2018 |
| Neon Collective Limited | Information & communication | 2006 |
| Neon Punk Ltd | Wholesale & retail | 2017 |
| Neptune Life Limited | Financial services | 2022 |
| Neuropsychology Sussex Ltd | Health & social care | 2018 |
| New Lawns Sussex Ltd | Administrative & support | 2020 |
| NG Colwel Ltd | Financial services | 2025 |
| NH Case Management Ltd | Professional & scientific | 2022 |
| Nh&Sh Safety Ltd | Professional & scientific | 2021 |
| Nick Cudworth Gallery Ltd | Wholesale & retail | 2023 |
| Nick Ryan Motor Works Limited | Wholesale & retail | 2014 |
| Nido Montessori Limited | Education | 2016 |
| NMI Certin UK (TIC) Limited | Professional & scientific | 2021 |
| NMI Metrology UK Holdco Limited | Financial services | 2023 |
| NO.1 Alfred Road Management Limited | Households | 1997 |
| Noircat Media Limited | Information & communication | 2026 |
| Northcroft (Brighton) Limited | Households | 2005 |
| Novus Controls Limited | Professional & scientific | 2011 |
| NRS Innovations Ltd | Wholesale & retail | 2009 |