| MAD Dog Apparel Limited | Wholesale & retail | 2024 |
| MAD Penguin Consulting Ltd | Information & communication | 2017 |
| Maddie RAE Developments Ltd | Real estate | 2022 |
| Maddison Squared Gardens & Waste Services Ltd | Water & waste | 2021 |
| Maddock Rentals Ltd | Real estate | 2019 |
| Madoduo International Trading Ltd | Wholesale & retail | 2024 |
| Maesycoed Stores Limited | Wholesale & retail | 2011 |
| Magic Dessert House Ltd | Hospitality | 2026 |
| Magnifix Luxe Events Ltd | Administrative & support | 2026 |
| Maguire Construction UK Limited | Construction | 1998 |
| Maguire Holdings Limited | Real estate | 1991 |
| Maguire Leisure Park Homes Limited | Financial services | 2025 |
| MAH82 Ltd | Transport & logistics | 2026 |
| Maidment Technical Services Ltd | Professional & scientific | 2025 |
| Maison Elora Limited | Wholesale & retail | 2025 |
| Major Hair Ltd | Wholesale & retail | 2025 |
| Major MOT & Services Centre Ltd | Wholesale & retail | 2024 |
| Make IT CNC Ltd | Manufacturing | 2020 |
| Malakai Estates Ltd | Real estate | 2016 |
| Mali Rail Limited | Construction | 2020 |
| Manage Money Wales C.I.C. | Education | 2015 |
| Mane Empire Limited | Other services | 2025 |
| Mango Hair Limited | Other services | 2023 |
| Mangoberry Ltd | Professional & scientific | 2022 |
| Manni GAO Ltd | Wholesale & retail | 2024 |
| Marble Arch Investment Group 2024 Limited | Financial services | 2024 |
| Marble Arch Investment Group Limited | Financial services | 2017 |
| Marcus Barber Shop Ltd | Other services | 2018 |
| MARHABA112 Ltd | Hospitality | 2025 |
| Mark COX Consultancy Limited | Professional & scientific | 2026 |
| Mark Furmage AE Services Limited | Health & social care | 2025 |
| Mark Goodson Surgical Assistance Ltd | Health & social care | 2023 |
| Marketing Purks Limited | Professional & scientific | 2020 |
| Marlan Properties Limited | Construction | 2024 |
| MARPRIO-10 Ltd | Wholesale & retail | 2025 |
| Marscott Ltd | Construction | 2017 |
| Martin Holmes Engineering Limited | Professional & scientific | 2020 |
| Master Painters & Decorators Limited | Construction | 2010 |
| Matchyourhome Ltd | Administrative & support | 2020 |
| Matrix TV Ltd | Information & communication | 2019 |
| Matt Trigg and Associates Ltd | Professional & scientific | 2017 |
| Mavericks Catering Ltd | Hospitality | 2022 |
| Mavron Limited | Wholesale & retail | 2026 |
| Maxim Sprinkler Holdings Limited | Financial services | 2021 |
| Maxim Sprinkler Systems Limited | Construction | 2017 |
| MAY James Properties Ltd | Real estate | 2022 |
| Maya Food Catering Ltd | Hospitality | 2026 |
| MC452 Ltd. | Education | 2008 |
| Mcdonnell Stores Ltd | Wholesale & retail | 2025 |
| MCL Flooring Limited | Construction | 2005 |
| Mdfoods Ltd Wales CYF. | Hospitality | 2022 |
| Mdinn IEE Ltd | Wholesale & retail | 2025 |
| MDP Retail (Llanelli) Ltd | Wholesale & retail | 2012 |
| MDP Retail (Nuneaton) Limited | Wholesale & retail | 2010 |
| MDP Retail (Swindon) Ltd | Wholesale & retail | 2011 |
| MDS Contractors Ltd | Construction | 2008 |
| Meadowbank Labels Limited | Wholesale & retail | 2005 |
| Meddev Consultancy Limited | Professional & scientific | 2023 |
| MEDGO2U Limited | Information & communication | 2025 |
| Medley Swim School Limited | Education | 2018 |
| Medora Investment Ltd | Real estate | 2023 |
| Meecham Surgical Services Limited | Health & social care | 2022 |
| MEG (South Wales) Ltd | Manufacturing | 2013 |
| Meglyn Music Ltd | Education | 2017 |
| Mejin Hair Ltd | Wholesale & retail | 2025 |
| Mekoms Ltd | Professional & scientific | 2024 |
| Memorabilia South Wales CYF | Wholesale & retail | 2022 |
| Mena Premier Holdings Limited | Real estate | 2025 |
| Mendocino Dream Ltd | Wholesale & retail | 2020 |
| Mengkai Fashion Company Ltd | Wholesale & retail | 2026 |
| Meridian Services Group Ltd. | Extraterritorial | 2023 |
| Merlin Engineering (Pontypridd) Limited | Manufacturing | 1995 |
| Merthyr Road 1971 Limited | Extraterritorial | 2014 |
| Meylee International Co., Ltd | Professional & scientific | 2025 |
| Micro Acres Ltd | Manufacturing | 2021 |
| MID Glamorgan Drylining Ltd | Construction | 2022 |
| MID Glamorgan Fork Truck Training Services Ltd | Transport & logistics | 2002 |
| Mifuture Group Ltd | Information & communication | 2015 |
| Mighty Signs Ltd | Manufacturing | 2019 |
| Mikdan Properties Ltd | Construction | 2012 |
| Mike & Mary Limited | Hospitality | 2019 |
| Mike Jones (Carpentry) Limited | Construction | 2002 |
| Mike Watkins Estate Agents Ltd | Real estate | 2019 |
| Milan Enterprise Limited | Professional & scientific | 2015 |
| Miles White Transport Limited | Transport & logistics | 2018 |
| Milf Limited | Professional & scientific | 2004 |
| Millard AX Consultants Limited | Information & communication | 2013 |
| Mima Food Limited | Hospitality | 2017 |
| Mirac Homes Ltd | Real estate | 2026 |
| Mirth Group Limited | Public administration | 2022 |
| Miskin Van Centre Limited | Wholesale & retail | 2018 |
| MJ & Sons PVT Ltd | Wholesale & retail | 2024 |
| MJ Davies Carpentry & Building Limited | Construction | 2025 |
| MJD Commercial Limited | Construction | 2021 |
| MJM Decorators Limited | Construction | 2002 |
| MKF Group Limited | Real estate | 2025 |
| MKO Automation Limited | Manufacturing | 2025 |
| MKO BMS Controls Ltd | Construction | 2026 |
| MLP Properties (UK) Ltd | Real estate | 2013 |
| MLS Building & Construction Limited | Construction | 2016 |