| MFB Contractors Ltd | Construction | 2024 |
| Mfkl Properties Limited | Real estate | 2020 |
| Mforce Properties Limited | Real estate | 2022 |
| Mforce Solutions Limited | Professional & scientific | 2021 |
| MG Phoenix Holding Limited | Financial services | 2017 |
| MHK Medics Care LLP | | 2026 |
| MHK Medics Ltd | Health & social care | 2011 |
| MHT Healthcare Limited | Health & social care | 2022 |
| MI Barrio Hitchin Ltd | Hospitality | 2025 |
| Michael Collins Architect LLP | | 2017 |
| Michael Collins Architects (Hertfordshire) Limited | Professional & scientific | 2025 |
| Michael Dales Partnership Limited | Professional & scientific | 2003 |
| Michael Rushton Consulting Limited | Extraterritorial | 2025 |
| MID Kent Scaffolding Ltd | Construction | 1998 |
| Middleton Medicolegal Limited | Professional & scientific | 2019 |
| Midshires Equestrian Massage Therapy Ltd | Health & social care | 2022 |
| Mighty EVE Ltd | Wholesale & retail | 2020 |
| Mighty Minis & Mums Ltd | Education | 2025 |
| Mihbash Ltd | Hospitality | 2026 |
| MII Papa Limited | Education | 2024 |
| Mike Dawson Antimicrobial Research Consultancy Limited | Professional & scientific | 2017 |
| Mike Harris Limited | Arts & recreation | 2019 |
| Mike Roberts Consultancy Limited | Professional & scientific | 2012 |
| Mikey O'S T,T&F Ltd | Information & communication | 2018 |
| Miles Film & TV Services Limited | Arts & recreation | 2012 |
| Milkitpr Limited | Professional & scientific | 2015 |
| Milkys Auto-Repair Ltd | Wholesale & retail | 2024 |
| Mill Farm Events Ltd | Hospitality | 2020 |
| Millea Jewellery Ltd | Wholesale & retail | 2026 |
| Miller Renewables Limited | Professional & scientific | 2016 |
| Millfield Lane Guardians Limited | Households | 2014 |
| Mills Building Services Surveyors LLP | | 2007 |
| Milta Homes Ltd | Real estate | 2023 |
| Milton House Nursing & Care Limited | Health & social care | 2019 |
| Minchneat Services Ltd | Public administration | 2021 |
| Mind the GAP Training & Consultancy Limited | Professional & scientific | 2020 |
| Mindful Collectives Limited | Wholesale & retail | 2018 |
| Minhan Conmin Trading Limited | Wholesale & retail | 2026 |
| Minhas Consulting Limited | Real estate | 2019 |
| Minivale (Hitchin) Limited | Construction | 1971 |
| Minnie Holdings Ltd | Real estate | 2022 |
| Miraism Limited | Other services | 2010 |
| Mirissa's Pantry Ltd | Hospitality | 2025 |
| Mirror Finish Limited | Manufacturing | 2002 |
| Misenum Ltd | Hospitality | 2018 |
| Misfits Operations Limited | Hospitality | 2025 |
| Misplodger Ltd | Real estate | 2023 |
| Miss Darcie Ltd | Financial services | 2021 |
| Mission Direct Limited | Other services | 2004 |
| Mister 4X4 Limited | Wholesale & retail | 2023 |
| Mistry Cyber Consultancy Ltd. | Information & communication | 2022 |
| Mitch & Scott Limited | Administrative & support | 2026 |
| Mitchell Educational Psychology Ltd | Education | 2021 |
| Mito Group Limited | Financial services | 2025 |
| Mito Land Limited | Real estate | 2024 |
| Mito Planning & Urban Design Limited | Other services | 2025 |
| MIX Partners Limited | Administrative & support | 2023 |
| MJK Partners Limited | Professional & scientific | 2018 |
| MJL Communications Group Ltd | Financial services | 2021 |
| MJM Autos Limited | Wholesale & retail | 2005 |
| Mjmc Electrical Contractors Ltd | Construction | 2020 |
| MK Cars Auto Dismantlers Ltd | Wholesale & retail | 2019 |
| Mkba Herts Ltd | Hospitality | 2023 |
| Mkcars Breakers Limited | Wholesale & retail | 2024 |
| Mkcs Properties Ltd | Real estate | 2007 |
| MKE Architecture Ltd | Professional & scientific | 2023 |
| MKF Services Limited | Transport & logistics | 2015 |
| Mktg AGE Ltd | Information & communication | 2026 |
| ML Chappory Ltd | Water & waste | 2026 |
| Mlait Renovation & Maintenance Ltd | Construction | 2025 |
| MLG Garden Works Ltd | Administrative & support | 2022 |
| MLO Electrical Limited | Construction | 2022 |
| MM Lettings Ltd | Real estate | 2013 |
| Mmav Management Ltd | Real estate | 2026 |
| MML Employment Ltd | Administrative & support | 2023 |
| MML Group Holdings Ltd | Administrative & support | 2023 |
| MMT Investments Limited | Real estate | 2018 |
| Mnifst Ltd | Information & communication | 2023 |
| MNK Consulting Limited | Information & communication | 2022 |
| Moami THS Ltd | Information & communication | 2020 |
| Moat Manor House Limited | Real estate | 2019 |
| Mobile Munchies Group Ltd | Hospitality | 2015 |
| Model Intelligence Limited | Professional & scientific | 2007 |
| Modern Maker Limited | Other services | 2007 |
| Modern Networks Limited | Information & communication | 1999 |
| Modern Networks Services Limited | Information & communication | 2008 |
| Modern Property Development Limited | Real estate | 2004 |
| Modern Property Management Limited | Professional & scientific | 2021 |
| Modern Salon Solutions Ltd | Wholesale & retail | 2025 |
| Modern-Id Limited | Wholesale & retail | 2016 |
| Modoofy Limited | Professional & scientific | 2024 |
| Modulabs Ltd | Professional & scientific | 2026 |
| Modumoti Limited | Hospitality | 2018 |
| Molecular Economics Limited | Professional & scientific | 2025 |
| Momentum Filmworks Limited | Other services | 2025 |
| Mondial Travel Limited | Administrative & support | 2018 |
| Money Insight Experts Limited | Administrative & support | 2025 |
| Money Means Limited | Financial services | 2021 |
| Monkey’S Emporium Herts Ltd | Wholesale & retail | 2016 |
| Monotheist Holdings Limited | Financial services | 2020 |