| Monarty Studio LAB Limited | Arts & recreation | 2022 |
| Money Badassary Limited | Education | 2020 |
| Monita Cheung Design Limited | Professional & scientific | 2014 |
| Monk Sherborne House Management Company Limited | Households | 1971 |
| Montauk Lifestyle Limited | Manufacturing | 2022 |
| Montclare House (Eastbourne) Management Company Limited | Households | 2002 |
| Montclare House (Eastbourne) RTM Limited | Households | 2006 |
| Monterey Wharf (Eastbourne) Limited | Other services | 1986 |
| Monterey Wharf RTM Company Limited | Other services | 2013 |
| Montessori Home School Hub Ltd | Education | 2025 |
| Montpelier Mansions Limited | Households | 1981 |
| Montpezat Limited | Construction | 2000 |
| Monty Capital Limited | Financial services | 2018 |
| Moonrise Bakery and Deli Ltd | Manufacturing | 2019 |
| Moonshine Billy's Limited | Wholesale & retail | 2015 |
| Moores Gypsy Cobs and General Dealers S/East Limited | Agriculture & fishing | 2016 |
| Moose & PIE Studios Ltd | Information & communication | 2025 |
| Moport Limited | Administrative & support | 2010 |
| More Days Limited | Information & communication | 2024 |
| More for Your Money LLP | | 2026 |
| More Than a Kitchen Ltd | Hospitality | 2023 |
| Morehen Architects Ltd. | Professional & scientific | 2015 |
| Moreview Properties Limited | Real estate | 2008 |
| Morgan Smiles Ltd | Health & social care | 2011 |
| Morgans Wine Bar Ltd | Hospitality | 2026 |
| Morrison Entwisle Radiology Limited | Health & social care | 2011 |
| Morrison Productions Ltd | Wholesale & retail | 2023 |
| Morson Social Work Solutions Ltd | Health & social care | 2012 |
| Mosaic Dental Healthcare Limited | Financial services | 2021 |
| Mosaic Dental Limited | Health & social care | 2020 |
| Mosaic Dental Property Limited | Health & social care | 2020 |
| Moss House Limited | Extraterritorial | 1959 |
| MOT Shop (Brighton) Limited | Wholesale & retail | 2013 |
| MOT Solutions Limited | Wholesale & retail | 2011 |
| Motcombe Community Pool CIC | Public administration | 2022 |
| Motcombe Cottage Gardens Ltd | Real estate | 2017 |
| Motcombe Court RTM Company Limited | Households | 2011 |
| Motcombe Lane (Eastbourne) Management Co. Ltd | Professional & scientific | 2000 |
| Motcombe Motors Ltd | Wholesale & retail | 2021 |
| Motherboard Ltd | Administrative & support | 2021 |
| Motion Physiotherapy UK Ltd | Health & social care | 2025 |
| Motion Picture Licensing Company (International) Limited | Other services | 1993 |
| Motion Picture Licensing Company Limited | Other services | 2012 |
| Motion to Move Ltd | Information & communication | 2011 |
| Motion Transport Services Ltd | Transport & logistics | 2026 |
| Moto Broker Limited | Wholesale & retail | 2019 |
| Mount Tenants Association (Eastbourne) Limited (the) | Households | 1972 |
| Mountain Fold Architecture Limited | Professional & scientific | 2018 |
| Mountbatten Programmes Limited | Education | 1988 |
| Mountford Holdings Limited | Real estate | 2021 |
| Mouse & Wren Ltd | Professional & scientific | 2026 |
| Movement Properties Limited | Households | 2000 |
| Moving Pictures Media Ltd | Administrative & support | 2009 |
| Mpamhood North Limited | Wholesale & retail | 2022 |
| MPB Dental Limited | Health & social care | 2025 |
| MPF Scaffold Ltd | Construction | 2017 |
| MPG Property Consultants Limited | Professional & scientific | 2023 |
| MPP Electrical Contractors Ltd | Construction | 2025 |
| MPS London Ltd | Information & communication | 2016 |
| MR Chippy Eastbourne Limited | Hospitality | 2025 |
| MR Distributions Ltd | Transport & logistics | 2025 |
| MR. Duvet Limited | Other services | 1996 |
| Mraq Limited | Health & social care | 2021 |
| MRC Salon Services Limited | Other services | 2014 |
| MRE Fish & Chips Ltd | Hospitality | 2025 |
| Mrshanbrown Limited | Professional & scientific | 2024 |
| MS Ritu Kalsi Ltd | Other services | 2013 |
| MS Roofing EB Limited | Construction | 2020 |
| Mshm Services Limited | Real estate | 2019 |
| Msjw Holdings Limited | Financial services | 2025 |
| MSM Mobiles Limited | Wholesale & retail | 2024 |
| Msquared IT Storage & Solutioning Ltd | Information & communication | 2023 |
| MTC Surveying Solutions Limited | Professional & scientific | 2021 |
| MTJ Property Ltd | Real estate | 2023 |
| Mtja Ltd | Education | 2025 |
| MTM Medics Ltd | Health & social care | 2023 |
| Mungeam & Co Properties Ltd | Real estate | 2019 |
| Murphy Property Group Ltd | Real estate | 2025 |
| Musabi Ltd | Health & social care | 2026 |
| Muscle Power Ltd | Wholesale & retail | 2017 |
| Music Marketing Ltd | Arts & recreation | 2021 |
| Musk C Limited | Wholesale & retail | 2018 |
| Musket Limited | Administrative & support | 2005 |
| Muslim Community of East Sussex - Al-Khidmat C.I.C. | Other services | 2024 |
| Mustard Seed Homes Limited | Real estate | 2020 |
| Mustard Seed Hub Limited | Professional & scientific | 2022 |
| Mutant Nation Ltd | Information & communication | 2011 |
| MVM2 Limited | Health & social care | 2013 |
| MVM3 Limited | Real estate | 2022 |
| MVM4 Limited | Health & social care | 2023 |
| MW Image Rights Ltd | Professional & scientific | 2023 |
| Mwbm Ltd | Professional & scientific | 2020 |
| My Accounts Digital Limited | Professional & scientific | 2021 |
| My Barbers (Eastbourne) Limited | Other services | 2019 |
| My Block Manager Ltd | Information & communication | 2023 |
| My Corporate Flat Limited | Hospitality | 2017 |
| My Daily Bread Ltd | Manufacturing | 2018 |
| My Emotion Coach Limited | Education | 2025 |
| My Fertility GP Ltd | Health & social care | 2021 |
| My Giddy Aunt Limited | Wholesale & retail | 2025 |