| Mahtab Hilton Swansea Ltd | Hospitality | 2016 |
| MAI Chai Holdings Limited | Financial services | 2025 |
| MAI Chai Ltd | Manufacturing | 2023 |
| MAI Motor Repairs Ltd | Manufacturing | 2016 |
| Maid in the Philippines Limited | Administrative & support | 2018 |
| Maidenstone Consulting Ltd | Professional & scientific | 2026 |
| Maidenstone Holdings Ltd | Real estate | 2026 |
| Maily Builders Limited | Construction | 2019 |
| Main's Martial Arts Academy Ltd | Education | 2020 |
| Mainshare Limited | Financial services | 2003 |
| Mainstream MMA Limited | Extraterritorial | 2011 |
| Mainwarings MTB Limited | Wholesale & retail | 2024 |
| Mainwarings MTB Outlet Limited | Wholesale & retail | 2025 |
| Mainway Fitness Ltd | Arts & recreation | 2019 |
| Maisam Trading & Shpping Ltd | Wholesale & retail | 2022 |
| Maisan Limited | Health & social care | 2012 |
| Maison DE Kimia Ltd | Other services | 2024 |
| Maison Marketing Ltd | Administrative & support | 2026 |
| Maison Triad Limited | Real estate | 2025 |
| MAJ Services Ltd | Administrative & support | 2026 |
| Majasala Ltd | Professional & scientific | 2023 |
| Maje Properties & Investments Ltd | Real estate | 2013 |
| Majestic Media Ltd | Information & communication | 2012 |
| Majestik (Cymru) Limited | Financial services | 2022 |
| MAK Surgicals Limited | Health & social care | 2014 |
| Makawood Limited | Manufacturing | 2025 |
| Make Good Decisions Ltd | Administrative & support | 2021 |
| Make the Change Consultancy Ltd | Professional & scientific | 2021 |
| Make.Surf Community Interest Company | Manufacturing | 2021 |
| Makeable Limited | Hospitality | 1997 |
| Makeovers With Mols Ltd | Other services | 2023 |
| Makers of Dance Energy Limited | Arts & recreation | 2003 |
| Makhuza Limited | Health & social care | 2024 |
| Making Tax Digital for YOU Limited | Professional & scientific | 2021 |
| Makmedico Ltd | Health & social care | 2024 |
| Makz Limited | Manufacturing | 2006 |
| Malcolm Lippiatt Homes Limited | Construction | 2006 |
| Malkaz Ltd | Real estate | 2025 |
| Mallard (Wales) Limited | Real estate | 2002 |
| Mallorca Home Search Limited | Real estate | 2021 |
| Mallorn Accounting Services Limited | Professional & scientific | 2013 |
| Maloney Properties Limited | Real estate | 2022 |
| Malsters Arms Limited | Hospitality | 2024 |
| Malton Lodge Ltd | Financial services | 2025 |
| Maltstate Limited | Real estate | 1995 |
| Maluga Ltd | Hospitality | 2021 |
| Malvern Walks Limited | Arts & recreation | 2011 |
| Mamaris Ltd | Administrative & support | 2019 |
| Mami's Fish & Chips Ltd | Hospitality | 2023 |
| Mammas Pizza Parlour Limited | Hospitality | 2019 |
| MAN About the House Ltd | Construction | 2022 |
| MAN and Van Specialist Ltd | Transport & logistics | 2023 |
| MAN Deliveries Limited | Transport & logistics | 2024 |
| Manasseh Health Care Ltd | Health & social care | 2022 |
| Mandible Projects Limited | Construction | 2025 |
| Mandrel Communications Limited | Professional & scientific | 2009 |
| Mangostorm Ltd | Professional & scientific | 2009 |
| Manmane Limited | Wholesale & retail | 2013 |
| Manny Holdings Ltd | Financial services | 2021 |
| Manny Renovations Ltd | Real estate | 2026 |
| Manolo Beauty Ltd | Other services | 2024 |
| Manor Park Country House Limited | Hospitality | 1999 |
| Manor Road Dental Practice Limited | Health & social care | 2007 |
| Manorek Homes Ltd | Real estate | 2025 |
| Mansel Express Ltd | Wholesale & retail | 2025 |
| Mansellz Cleaning Limited | Administrative & support | 2021 |
| Manselton 1959 Limited | Extraterritorial | 2014 |
| Mansion Gardens Limited | Households | 2019 |
| Manual Gearbox Repairs Limited | Wholesale & retail | 2019 |
| Manva Research Ltd | Professional & scientific | 2026 |
| Manzoautomotive Limited | Administrative & support | 2025 |
| MAP Engineering Swansea Limited | Manufacturing | 2010 |
| Maple Leaf Medical Centre Ltd | Health & social care | 2021 |
| Maps Auto Parts Ltd | Wholesale & retail | 2026 |
| Maps Auto Recovery Ltd | Wholesale & retail | 2022 |
| Maps Logistics Ltd | Wholesale & retail | 2020 |
| Mapuleng Ltd | Health & social care | 2014 |
| Marble Moon Management Company Limited | Households | 2000 |
| Marcelle Properties Ltd | Real estate | 2021 |
| Marcelline Technology Ltd | Wholesale & retail | 2024 |
| Marchnata Limited | Professional & scientific | 2024 |
| Marcus Directs Limited | Arts & recreation | 2014 |
| Marcus Technical Solutions Ltd | Construction | 2019 |
| Mardi Gras Ltd | Wholesale & retail | 2005 |
| Mardy MOT and Repair Center Limited | Wholesale & retail | 2023 |
| Marguerite Bay Limited | Professional & scientific | 2024 |
| Marguerite Williams Limited | Wholesale & retail | 2020 |
| Marha Design Ltd | Professional & scientific | 2017 |
| Maria Merlas Limited | Public administration | 2023 |
| Marija Developments Ltd | Professional & scientific | 2020 |
| Marina Investment Holdings Ltd | Professional & scientific | 2010 |
| Marina Overseas Investment (Holding) Limited | Administrative & support | 2013 |
| Marina United Ltd | Health & social care | 2010 |
| Marine 16 Ltd | Transport & logistics | 2004 |
| Marine Power Systems Ltd | Manufacturing | 2008 |
| Mariposa Nature Tours Ltd | Administrative & support | 2023 |
| Marivel V Care Limited | Health & social care | 2026 |
| Mark 88 Property Limited | Construction | 2021 |
| Mark Craven Property Maintenance Limited | Administrative & support | 2016 |
| Mark Eley Ground Maintenance Limited | Administrative & support | 2002 |