| M. Installation Limited | Wholesale & retail | 2008 |
| M. Rees Developments Limited | Construction | 2004 |
| M.A.M Global Ltd | Professional & scientific | 2024 |
| M.E.T. Inspection & Consultancy Ltd | Professional & scientific | 2015 |
| M.S.B. Productions Ltd | Administrative & support | 2021 |
| M.U.D Civil Engineering Ltd | Construction | 2026 |
| Mable's Food Co Ltd | Hospitality | 2026 |
| MAC 17 Ltd | Construction | 2024 |
| Mach 7-7 Group Ltd | Professional & scientific | 2012 |
| Machurst Development's Ltd | Construction | 2018 |
| Mackey Demolition Limited | Water & waste | 2015 |
| Mackey Group Holdings Ltd | Water & waste | 2025 |
| Mackin Ltd | Professional & scientific | 2026 |
| Maclock Limited | Administrative & support | 2015 |
| Macmillan Power Limited | Energy | 2013 |
| Macob Commercial Limited | Construction | 2021 |
| Macob Plant Limited | Administrative & support | 2022 |
| Macob Westgate Limited | Construction | 1998 |
| Macosie Properties Limited | Real estate | 2017 |
| MAD2 Limited | Real estate | 2012 |
| Madison Gardens Limited | Information & communication | 2003 |
| Madness Events Ltd | Arts & recreation | 2023 |
| Maehr Consulting Limited | Health & social care | 2015 |
| Maes Y Parc (Glynneath) Management Company Limited | Households | 2022 |
| Maesteg Celtic Bowling Club Ltd | Arts & recreation | 2020 |
| Maesteg Pharmacy Limited | Health & social care | 2003 |
| Magenta Financial Planning Limited | Financial services | 2016 |
| Magenta Group Holdings Limited | Professional & scientific | 2020 |
| Magentamen Ltd | Professional & scientific | 2012 |
| Magnolia Parkfields Limited | Administrative & support | 2024 |
| Maia Partners Limited | Professional & scientific | 2022 |
| Mainedge Limited | Extraterritorial | 2025 |
| Major Cars Ltd | Information & communication | 1998 |
| Making Lean Work Ltd | Professional & scientific | 2009 |
| Mali Consulting Limited | | 2014 |
| Malikson Ltd | Hospitality | 2018 |
| Mancharles Hair & Grooming Ltd | Other services | 2021 |
| Mancharles Hair Ltd | Other services | 2025 |
| Manderssen Access Ltd | Financial services | 2025 |
| Mango Planning & Development Limited | Professional & scientific | 2011 |
| Manjoro Buller Ltd | Health & social care | 2023 |
| Manning Construction Limited | Construction | 2002 |
| Mansa Security Limited | Administrative & support | 2025 |
| Mansels Coity Ltd | Hospitality | 2023 |
| Mansels Fish and Chip Shop Ltd | Hospitality | 2020 |
| Mansels Porthcawl Limited | Hospitality | 2025 |
| Mansys ISS Holdings Limited | Administrative & support | 2018 |
| Mansys ISS Ltd | Administrative & support | 2015 |
| Mantaray Executive Resourcing Ltd. | Administrative & support | 2011 |
| Manuel Property Management Ltd | Real estate | 2024 |
| Maplemore Limited | Professional & scientific | 2018 |
| Marbast Limited | Manufacturing | 2011 |
| Marble Steakhouse Ltd | Hospitality | 2022 |
| Mariachi Loco (UK) Limited | Arts & recreation | 2016 |
| Marine Beauty Ltd | Wholesale & retail | 2024 |
| Mariojity Transport Ltd | Transport & logistics | 2021 |
| Mark Hudson Consulting Ltd | Information & communication | 2014 |
| Mark Rapley Limited | Construction | 2014 |
| Mark Thomas Property Improvements Limited | Construction | 2017 |
| Markdeed Limited | Extraterritorial | 2025 |
| Markes Analytics Solutions Limited | Financial services | 2018 |
| Markes International Holdings Limited | Professional & scientific | 2012 |
| Markes International Limited | Manufacturing | 1997 |
| Marlas Road 1965 Limited | Extraterritorial | 2014 |
| Marod Homes Limited | Real estate | 2020 |
| Marriott Properties Lettings Limited | Real estate | 2023 |
| Marshoval Limited | Wholesale & retail | 2020 |
| Martha ANN Elizabeth Martin Limited | Extraterritorial | 2023 |
| Marxen Ltd | Hospitality | 2024 |
| Mary Matha Enterprises Ltd | Wholesale & retail | 2019 |
| Mase Electrical Ltd | Construction | 2018 |
| Mason Force Media Ltd | Information & communication | 2024 |
| Masoneve Limited | Extraterritorial | 2025 |
| Masonpalm Limited | Extraterritorial | 2026 |
| Masons Site Solutions Ltd | Construction | 2023 |
| Masserini Systems Ltd | Information & communication | 2026 |
| Mastbray Limited | Extraterritorial | 2025 |
| Mastering Diversity CIC | Education | 2024 |
| Matchflat Limited | Hospitality | 2004 |
| Matthew Tipper Limited | Arts & recreation | 2010 |
| Matthew Vending Ltd | Manufacturing | 2024 |
| Matthews & SON Plumbing & Heating Ltd | Construction | 2016 |
| Matthews Cleaning Services Limited | Households | 2006 |
| Matthews Commercial Cleaning Services Ltd | Households | 2011 |
| Maven Recruitment Solutions Ltd | Administrative & support | 2025 |
| Maverick Properties (SW) Limited | Real estate | 2024 |
| MAX Internet Shopping Ltd | Wholesale & retail | 1995 |
| Maxi Global Security Limited | Administrative & support | 2025 |
| Maxime Partnership Limited | Construction | 2013 |
| Maxitrac Limited | Manufacturing | 2019 |
| Maxx Bridgend Limited | Wholesale & retail | 2014 |
| Maya Motorworks Ltd | Wholesale & retail | 2020 |
| Maymara Medical (MMM) Ltd | Health & social care | 2019 |
| Maymax Engineering Limited | Manufacturing | 2003 |
| Maythagu Ltd | Hospitality | 2025 |
| Maytree Contracts Limited | Construction | 2003 |
| Mazuma GB Limited | Professional & scientific | 2008 |
| MBI (Wales) Limited | Manufacturing | 2006 |
| MC Properties (Bridgend) Limited | Real estate | 2025 |
| Mcallister Trading Limited | Wholesale & retail | 2025 |