- MAREDUDD ap CYNAN ab OWAIN GWYNEDD (d. 1212), lord of Eifionydd, part of Ardudwy, and Merioneth and co-founder of the Cistercian house of Cymmer
- MAREDUDD ap GRUFFYDD ap RHYS (1130 - 1155), prince of Deheubarth
- MAREDUDD ap IEUAN ap ROBERT ap MAREDUDD Gwydir (fl. end of 15th century) - see WYNN
- MAREDUDD ap MORGAN ap CARADOG ap IESTYN - see MORGAN ap CARADOG ap IESTYN
- MAREDUDD ap RHOSER (fl. c. 1530), poet
- MAREDUDD ap RHYS - see MEREDYDD ap RHYS
- MAREDUDD ap RHYS GRYG (d. 1271), prince of Deheubarth
- MARGED ferch IFAN (1696 - 1801?) - see MARGED vch IFAN
- MARGED vch IFAN (MARGED vch IFAN (Margaret Evans; 1696 - 1801?), 'character'
- MARI'R FANTELL WEN - see EVANS, MARY
- MARKS, DAVID (1788 - 1871), musician
- MARMORA - see MORRIS, DAVID WILLIAM
- MARQUAND, HILARY ADAIR (1901 - 1972), economist and Labour politician
- MARSDEN, THOMAS (1802 - 1849), cleric and author
- MARSH, JOHN (1747 - 1795), bookseller - see MARSH, RICHARD
- MARSH, RICHARD (1710? - 1792), bookseller and printer
- MARSHAL family (earls of Pembroke),
- MARSHAL, ANSELM (d. 1245) - see MARSHAL
- MARSHAL, GILBERT (d. 1241) - see MARSHAL
- MARSHAL, RICHARD (d. 1234) - see MARSHAL
- MARSHAL, WALTER (d. 1245) - see MARSHAL
- MARSHAL, WILLIAM (1146? - 1219), regent of England - see MARSHAL
- MARSHAL, WILLIAM (d. 1231) - see MARSHAL
- MARSHALL, WALTER CHARLES (1932 - 1996), scientist
- MARTHA'R MYNYDD, Mountain Martha (fl. c. 1770), impostor
- MARTIN, Sir RICHARD (1843 - 1922), industrialist and public man
- MARTIN, Major WILLIAM - see MICHAEL, GLYNDWR
- MARY FITT - see FREEMAN, KATHLEEN
- MASON, LILIAN JANE (1874 - 1953), actress
- MASON, RICHARD (1816? - 1881), printer and author
- MATCHLESS ORINDA - see PHILIPPS family of TREGYBI
- MATHAU GOCH - see GOUGH, MATHEW
- MATHETES - see JONES, JOHN
- MATHEW family Castell y Mynach,
- MATHEW ap LLYWELYN GOCH (fl. middle of the 16th century), poet
- MATHEWS, ABRAHAM - see MATTHEWS, ABRAHAM
- MATHEWS, ABRAHAM (1832 - 1899), Independent minister, colonist, and writer
- MATHEWS, THOMAS (1675 - 1751), M.P. - see MATHEW
- MATHIAS family Llwyngwaren, Llwyn Gwaring, Llangwaren, Lamphey
- MATHIAS, JAMES GORONWY (Goronwy Ddu; 1842 - 1895), Baptist minister and littérateur
- MATHIAS, RONALD CAVILL (1912 - 1968), trade union leader
- MATHIAS, WILLIAM JAMES (1934 - 1992), composer and teacher
- MATTAN, MAHMOOD HUSSEIN (1923 - 1952), seaman and victim of injustice
- MATTHEWS, ABRAHAM (1832 - 1899), minister (Congl.) and one of the pioneers of the Welsh settlement in Patagonia
- MATTHEWS, DANIEL HUGH (1936 - 2020), Baptist minister and college principal
- MATTHEWS, EDWARD (1813 - 1892), Calvinistic Methodist minister and author
- MATTHEWS, JOHN (1773 - 1848), surveyor and public man
- MATTHEWS, JOHN HOBSON (Mab Cernyw; 1858 - 1914), Roman Catholic historian, archivist and solicitor
- MATTHEWS, MARMADUKE (1606 - 1683?), ejected minister
- MATTHEWS, NORMAN GREGORY (1904 - 1964), chancellor