| M & P Trading (Essex) Limited | Wholesale & retail | 2007 |
| M and K Commerce Limited | Wholesale & retail | 2022 |
| M G Smith Plumbing & Heating Limited | Construction | 2010 |
| M Salmon Development Limited | Construction | 2023 |
| M&Y Garage Ltd | Wholesale & retail | 2026 |
| M.Bryant Painting & Decorating Services Limited | Construction | 2018 |
| M.D.A. (Marketing Services) Limited | Professional & scientific | 1978 |
| MAA Cosmetics Ltd | Wholesale & retail | 2023 |
| Macleans Farms (Essex) Limited | Agriculture & fishing | 1983 |
| Macs Group Holdings Ltd | Real estate | 2019 |
| Macscan Ltd | Information & communication | 2025 |
| Made in the Sky Limited | Financial services | 2016 |
| Made Mine Ltd | Manufacturing | 2017 |
| Magicpics UK Limited | Professional & scientific | 2011 |
| Mainline Sensors Ltd | Wholesale & retail | 2022 |
| Mains 'N' Drains Limited | Construction | 2008 |
| Malfy Drylining & Interiors Ltd | Construction | 2023 |
| Mamba Marketing Consulting Limited | Professional & scientific | 2017 |
| Mangiare ELY Limited | Hospitality | 2022 |
| Mangiare Limited | Hospitality | 2017 |
| Mann Plumbing and Heating Limited | Construction | 2005 |
| Manson Mouldings Ltd | Manufacturing | 2021 |
| Many Purple Hats Ltd | Information & communication | 2021 |
| Maple Lane Developments Limited | Real estate | 1993 |
| Maplestead Farm Limited | Agriculture & fishing | 2019 |
| Marcam House Ltd | Real estate | 2022 |
| Marchesi Pavement Solutions Limited | Professional & scientific | 2019 |
| Mareap Limited | Transport & logistics | 2025 |
| Marites Care Ltd | Administrative & support | 2025 |
| Mark 1 Boiler Services Limited | Construction | 2011 |
| Mark Hopwood Photography Ltd | Professional & scientific | 2009 |
| Marshall Swimming Pools Ltd | Construction | 2020 |
| Martin Builders Limited | Construction | 2007 |
| Martin Simmonds Ltd | Arts & recreation | 2023 |
| Martin's Lane Estates (Stow Maries) LLP | | 2008 |
| Martyn J Bailey Luthier Limited | Other services | 2002 |
| Mary ANN Tuli Ltd | Information & communication | 2017 |
| Mason Documents Limited | Administrative & support | 2015 |
| Mason Fang Ltd | Wholesale & retail | 2025 |
| Master Craftsmen Kitchens Limited | Wholesale & retail | 2012 |
| Match Day Centres Ltd | Arts & recreation | 2010 |
| Matt Mackman Ltd | Administrative & support | 2016 |
| Matthew Chater Associates Limited | Construction | 2001 |
| Matthew Corporation Limited | Administrative & support | 2002 |
| Matthew Holdcroft Limited | Professional & scientific | 2014 |
| Maurice Lake & Co Limited | Professional & scientific | 2002 |
| MAX Tune Limited | Wholesale & retail | 2021 |
| MAX Tune Motorsport Limited | Arts & recreation | 2025 |
| MAY Logistics Ltd | Transport & logistics | 2026 |
| Maybank Associates Limited | Construction | 2012 |
| Maylin Consulting Limited | Professional & scientific | 2018 |
| MBC Investment Management Ltd | Real estate | 2008 |
| MBC Investment Property Management Ltd | Administrative & support | 2009 |
| MCD Building Services Ltd | Construction | 2018 |
| MCS Engineering (Colchester) Ltd | Professional & scientific | 2008 |
| MDS Civil Engineering Limited | Construction | 2000 |
| MDS Enterprises Limited | Financial services | 2011 |
| Meadowview Care Limited | Health & social care | 2009 |
| Medinger Associates Limited | Professional & scientific | 2009 |
| Mediproperties Limited | Real estate | 2012 |
| Membrane Testing Solutions Limited | Professional & scientific | 2014 |
| Meridian Chartware Ltd | Extraterritorial | 1995 |
| Merryfield Developments Limited | Construction | 2019 |
| Mersea Glass Systems Ltd | Administrative & support | 2026 |
| Mersea Island Building Company Ltd | Construction | 2019 |
| Mesh Lettings Limited | Real estate | 2024 |
| Metals Focus Data Limited | Professional & scientific | 2013 |
| Metcalfe Technologies Limited | Information & communication | 2016 |
| MFC Carpentry Ltd | Construction | 2021 |
| Michael John Services Ltd | Administrative & support | 2018 |
| Mick Cross Carpentry & Building Services Ltd | Construction | 2017 |
| Midlands Sushi Co Limited | Hospitality | 2024 |
| Mike Groves Consulting Limited | Professional & scientific | 2012 |
| Mileham Properties Limited | Real estate | 2017 |
| Milestones Healing Therapies Ltd | Health & social care | 2023 |
| Milldale Contracts Limited | Construction | 2013 |
| Mirror Mirror Halstead Ltd | Other services | 2024 |
| Mitpans Limited | Professional & scientific | 2017 |
| MIX & Match Carpets Limited | Wholesale & retail | 2003 |
| MJJ Management Services Limited | Professional & scientific | 2018 |
| MJW Stocktaking Limited | Administrative & support | 2015 |
| MK 2 Ltd | Manufacturing | 2022 |
| Modern Antics Ltd | Wholesale & retail | 2020 |
| Modern Living Property Solutions Limited | Real estate | 2022 |
| Moorcroft Property Developers Ltd | Construction | 2019 |
| Morley Occupational Therapy and Case Management Ltd | Health & social care | 2022 |
| Morningstar Enterprises Ltd | Administrative & support | 2021 |
| Mortimer Contracts Limited | Administrative & support | 2015 |
| Mortlock Brothers (Toppesfield) Limited | Agriculture & fishing | 1973 |
| Morton Projects Limited | Real estate | 2015 |
| MOT Services (Halstead) Limited | Wholesale & retail | 2002 |
| MOT Services Limited | Extraterritorial | 2024 |
| Mother Inked Ltd | Manufacturing | 2025 |
| Mothers of Goodness (Ummul Khair) Ltd | Health & social care | 2022 |
| Mouthful Multi Cuisine Ltd | Hospitality | 2019 |
| Moxley Property Group Ltd | Administrative & support | 2025 |
| MPH Contracts Limited | Construction | 2012 |
| MPH Design Limited | Administrative & support | 2006 |
| MPM Capital Limited | Wholesale & retail | 2026 |
| MPS Carpentry Contractors Limited | Construction | 2006 |