| Mansion House (Stonehouse) Management Limited | Households | 1997 |
| Mansys Solutions Limited | Other services | 2013 |
| Manta Hydrographic Services Ltd | Information & communication | 2026 |
| Manticore Spas Limited | Arts & recreation | 2013 |
| Manuclinical Ltd | Construction | 2020 |
| Manufakturo Ltd | Manufacturing | 2026 |
| Manumit Apparel Ltd | Wholesale & retail | 2023 |
| Manutek Ltd | Arts & recreation | 2019 |
| MAP Medical Logistics Ltd | Transport & logistics | 2020 |
| Maplestar Properties Limited | Real estate | 2002 |
| Marama Care PVT Ltd | Health & social care | 2012 |
| Marama Supported Living CIC | Hospitality | 2021 |
| Marandor Investments Ltd | Construction | 2022 |
| Marc Cole Property Services Limited | Construction | 2015 |
| Marc Lawson & Co Limited | Professional & scientific | 2005 |
| Marc Sealey Limited | Construction | 2009 |
| Marcel Motors Ltd | Wholesale & retail | 2017 |
| March Hair Plymouth Limited | Other services | 2007 |
| Marco UK (2005) Limited | Extraterritorial | 2005 |
| Maretec International Limited | Construction | 1981 |
| Margaret Mcmillan Limited | Other services | 2007 |
| Margaret's Care Ltd | Health & social care | 2022 |
| Margaux Luyt Limited | Professional & scientific | 2014 |
| Margo's Sandwich Shop Ltd | Hospitality | 2026 |
| Maria Comfort Ltd | Health & social care | 2019 |
| Mariapayne Limited | Wholesale & retail | 2026 |
| Marie Hewitt Interiors Ltd | Professional & scientific | 2016 |
| Marine & Industrial Profiles Limited | Energy | 1996 |
| Marine & Refit Solutions Ltd | Construction | 2016 |
| Marine AI Ltd | Professional & scientific | 2017 |
| Marine Civils Ltd | Construction | 2024 |
| Marine Claims Service Limited | Information & communication | 2020 |
| Marine Comply Ltd | Education | 2023 |
| Marine Engineering Services Ltd | Professional & scientific | 2024 |
| Marine Kinetics Ltd | Professional & scientific | 2026 |
| Marine Lifting & Safety Limited | Extraterritorial | 2011 |
| Marine Mining Research Limited | Extraterritorial | 2013 |
| Marine Projects (Plymouth) Limited | Extraterritorial | 1948 |
| Marine Report Solutions Ltd | Administrative & support | 2024 |
| Marine Sampling (UK) Ltd | Extraterritorial | 2021 |
| Marine Services (SW) Holdings Limited | Financial services | 2022 |
| Marine Solar Solutions Ltd | Manufacturing | 2026 |
| Marine Warranty Surveyors Limited | Professional & scientific | 2022 |
| Mariners Way Management Company Limited | Households | 1987 |
| Marion Distributor Ltd | Wholesale & retail | 2023 |
| Mariotrans Limited | Transport & logistics | 2015 |
| Marisco Refurbishments Ltd | Construction | 2017 |
| Mariska Smit Ltd | Health & social care | 2007 |
| Maristow Gardens Limited | Households | 1995 |
| Maritime Engineering Ltd | Professional & scientific | 2025 |
| Maritime Shipping Limited | Transport & logistics | 2010 |
| Maritime Tech Ltd | Manufacturing | 2019 |
| Maritimo Properties Limited | Hospitality | 2021 |
| Maritimo Trading Ltd | Hospitality | 2012 |
| Marjon (South West) Limited | Hospitality | 1989 |
| Mark Akerman Garage Services Ltd | Wholesale & retail | 2014 |
| Mark Bonning-Snook Ltd | Financial services | 2011 |
| Mark Bowden Limited | Health & social care | 2013 |
| Mark Christie Ltd | Real estate | 1998 |
| Mark Evans Planning Limited | Professional & scientific | 2014 |
| Mark German Financial Services Limited | Financial services | 2018 |
| Mark Holt & Co Holdings Limited | Professional & scientific | 2014 |
| Mark Holt & Co. Limited | Professional & scientific | 1997 |
| Mark John Consulting Ltd | Professional & scientific | 2026 |
| Mark Jones Building Limited | Construction | 2025 |
| Mark Lawson Design Limited | Professional & scientific | 2010 |
| Mark Moss Plumbing & Tiling Limited | Construction | 2023 |
| Mark Peters VAT Associates Limited | Professional & scientific | 2014 |
| Mark Rowe & SON LLP | | 2010 |
| Mark Venning Limited | Wholesale & retail | 2010 |
| MARK3D Improvements Limited | Construction | 2024 |
| Market 2 Media Ltd | Professional & scientific | 2020 |
| Market Crushers Ltd | Wholesale & retail | 2023 |
| Market Deli Limited | Wholesale & retail | 2003 |
| Market House Management Company (Kingsbridge) Limited | Real estate | 2012 |
| Market Square Investments Limited | Extraterritorial | 1980 |
| Market Square Limited | Hospitality | 1975 |
| Market Square Trading Limited | Hospitality | 2008 |
| Markus Green Products Limited | Wholesale & retail | 2019 |
| Marlands Consultancy Limited | Professional & scientific | 2021 |
| Marlborough Mews RTM Company Limited | Households | 2021 |
| Marley Mayer Plymouth Ltd | Other services | 2025 |
| Marmara Sterling UK Ltd | Wholesale & retail | 2024 |
| Marmaris Turkish Barbers Keyham Ltd | Other services | 2021 |
| Marney Health Ltd | Wholesale & retail | 2026 |
| Mars Orbit Limited | Real estate | 2019 |
| Marsden Therapy Hub CIC | Health & social care | 2026 |
| Marsean Transport Limited | Transport & logistics | 2003 |
| Marsh Mill Management Co. Limited | Administrative & support | 1986 |
| Marshall Commercial Properties Ltd | Real estate | 2026 |
| Marshall Lettings & Estates Ltd | Real estate | 2018 |
| Marshall Marine Services Ltd | Transport & logistics | 2018 |
| Marshall Parcel & Satellite Solution Ltd | Transport & logistics | 2018 |
| Marshall Strategic Land Ltd | Construction | 2018 |
| Marslands Accountants Limited | Professional & scientific | 2006 |
| Martarocon Consulting Ltd | Professional & scientific | 2023 |
| Martian Premier League Ltd | Information & communication | 2022 |
| Martin & Sons Monumental Masons Limited | Other services | 2012 |
| Martin Ebert Design Architecture Limited | Professional & scientific | 2018 |
| Martin Holiday Lettings Ltd | Real estate | 2016 |