| M. Reed Foot Trimming Limited | Wholesale & retail | 2013 |
| M.Wiltshire Moving & Storage Ltd | Administrative & support | 2017 |
| MAC Training and Education Ltd | Professional & scientific | 2011 |
| Maddox Property Investments Limited | Households | 2025 |
| Maes YR Eglwys Limited | Households | 2023 |
| Magdalen Ventures Limited | Financial services | 2021 |
| Magenta Coaching Ltd | Construction | 2020 |
| Maggie Rees Associates Ltd | Health & social care | 2009 |
| Maggs Optics Limited | Health & social care | 2002 |
| Magic Castle (GB) Limited | Arts & recreation | 1997 |
| Mahey Medics Limited | Health & social care | 2024 |
| Maid for ALL Pembrokeshire Ltd | Other services | 2018 |
| Mainport Maritime Management Limited | Real estate | 1988 |
| Maintain Values Ltd | Extraterritorial | 2023 |
| Maja International Ltd | Administrative & support | 2025 |
| Maleki Truck Holdings Group Ltd | Wholesale & retail | 2024 |
| Malosheo Ltd | Real estate | 2021 |
| Mamgu Welshcakes Limited | Manufacturing | 2018 |
| Manesa Ltd | Other services | 2021 |
| Mannan Umbrella Limited | Real estate | 2017 |
| Manomarine Ltd | Professional & scientific | 2023 |
| Mara's World Ltd | Administrative & support | 2021 |
| Maranel Ltd | Other services | 2023 |
| Marcus Reilly 51 Ltd | Construction | 2025 |
| Maria Grimwood Limited | Hospitality | 2019 |
| Mark Hunter Properties (Holdings) Limited | Financial services | 2024 |
| Mark Hunter Properties Limited | Financial services | 2024 |
| Marloes Conservation Limited | Professional & scientific | 2016 |
| Marloes Legal Limited | Professional & scientific | 2019 |
| Marlsbrough Farm Limited | Agriculture & fishing | 2003 |
| Martyn Corbet Music Ltd | Professional & scientific | 2017 |
| Masterfountain Limited | Agriculture & fishing | 2000 |
| Matheson Estates (Cornwall) Limited | Real estate | 1995 |
| Matthews Menswear Limited | Wholesale & retail | 2011 |
| Maxess Locks Ltd | Wholesale & retail | 2025 |
| Maxibon Ltd | Other services | 2021 |
| Mayho Consulting Limited | Professional & scientific | 2011 |
| Mccormack Maritime Service Ltd | Administrative & support | 2025 |
| Mcgeown's Super Whippy Properties Limited | Real estate | 2020 |
| Mcinn Ltd | Hospitality | 2017 |
| MDM Farms Limited | Agriculture & fishing | 2006 |
| Mdost Ltd | Other services | 2021 |
| MDP Electrical East Sussex Ltd | Construction | 2022 |
| Meadow Carpets Limited | Wholesale & retail | 2009 |
| Media to Motion Group Ltd | Information & communication | 2025 |
| Media to Motion Limited | Information & communication | 2016 |
| Meithrinfa DO RE MI Nursery CYF | Health & social care | 2020 |
| Melin Tregwynt Limited | Manufacturing | 2012 |
| Melin Tregwynt Trustee Limited | Professional & scientific | 2021 |
| Memory Clinic Wales Ltd | Health & social care | 2009 |
| Menevia Spa Ltd | Other services | 2018 |
| Merengue Moves Ltd | Real estate | 2025 |
| Merlins Magic Ltd | Arts & recreation | 2011 |
| Merod Ltd | Other services | 2022 |
| Merriman Farms Limited | Administrative & support | 2002 |
| Metal Seagulls Ltd | Professional & scientific | 2016 |
| Mhsquared Ltd | Professional & scientific | 2022 |
| MID and North Wales Air Ambulance Ltd | Extraterritorial | 2026 |
| Mike Howlin Motors Limited | Wholesale & retail | 2003 |
| Mikvan Limited | Public administration | 2023 |
| Mileade Construction Limited | Construction | 2020 |
| Miles John Williams Limited | Construction | 2022 |
| Miles Stott Children's Literary Agency Ltd | Information & communication | 2014 |
| Milestones Recruitment Ltd | Administrative & support | 2026 |
| Milford Haven Travel Agency Limited | Administrative & support | 1963 |
| Milking Machine Testing & Servicing Limited | Other services | 2007 |
| Mill Bay Homes Limited | Construction | 2017 |
| Millin Management Services Ltd | Professional & scientific | 2016 |
| Millmoor Management Company Limited | Professional & scientific | 2001 |
| Minerva Mindset Ltd | Professional & scientific | 2022 |
| Mingconsulting & Intuitionmanagement Ltd | Administrative & support | 2024 |
| Minhus UK Limited | Extraterritorial | 2015 |
| Mintpies Limited | Professional & scientific | 2020 |
| Miracomer Ltd | Transport & logistics | 2023 |
| Miss Molly Emma’S Ltd | Other services | 2025 |
| MJW Property Managment Ltd | Real estate | 2024 |
| MK Yunis Limited | Wholesale & retail | 2022 |
| Mlila Ltd | Other services | 2022 |
| MM Stonework and Construction Ltd | Manufacturing | 2025 |
| Moby Dicks Fish and Chips Ltd | Hospitality | 2020 |
| Moger & Associates Ltd | Hospitality | 2022 |
| Monix Fire Safety Ltd | Public administration | 2024 |
| Montrose Residential Home Limited | Health & social care | 2025 |
| Moor Farm Equestrian Centre Ltd | Administrative & support | 2001 |
| MOR Glas Property Management Limited | Administrative & support | 2012 |
| More Wealth Management Financial Planning Ltd | Professional & scientific | 2019 |
| Morlo Creative Ltd | Professional & scientific | 2019 |
| Morse Developments Limited | Real estate | 2021 |
| Mosaic Cloud Limited | Information & communication | 2010 |
| MOT at Autos Ltd | Wholesale & retail | 2022 |
| Motec Autos (Pembrokeshire) Limited | Wholesale & retail | 1998 |
| Motec Autos Pembrokeshire 2019 Ltd | Financial services | 2019 |
| Motonology Limited | Information & communication | 2020 |
| Motormatch Limited | Wholesale & retail | 2005 |
| Mountjoy Farms Limited | Agriculture & fishing | 2017 |
| Movement Matters Physiotherapy Limited | Other services | 2014 |
| MPE Contracting Ltd | Manufacturing | 2017 |
| MPH Hire Limited | Wholesale & retail | 2016 |
| MR Shift IT Pembs Ltd | Transport & logistics | 2015 |
| MR Shift IT Removals Pembrokeshire Ltd | Transport & logistics | 2025 |