- MAB CERNYW - see MATHEWS, JOHN HOBSON
- MAB Y CLOCHYDDYN (fl. c. 1380), poet
- MABON - see ABRAHAM, WILLIAM
- MACDONALD, GORDON (first Baron MACDONALD of GWAENYSGOR), (1888 - 1966), politician
- MACHEN, ARTHUR (1863 - 1947), writer
- MACHU Saint - see MALO, Saint
- MACHUTES Saint - see MALO, Saint
- MACKWORTH, CECILY JOAN (1911 - 2006), writer, poet, journalist and traveller
- MACKWORTH, Sir HUMPHREY (1657 - 1727), industrialist and parliamentarian
- MACLEAN, Sir EWEN JOHN (1865 - 1953), first professor of obstetrics and gynaecology at the Welsh National School of Medicine
- MACLOVIUS Saint - see MALO, Saint
- MADAM GRIFFITH - see GRIFFITH, SIDNEY
- MADDOCK, Sir IEUAN (1917 - 1988), Chief Scientist to the Department of Industry
- MADDOCKS, ANN (the Maid of Cefn Ydfa; 1704 - 1727)
- MADOC Saint - see AIDAN, Saint
- MADOCKS, WILLIAM ALEXANDER (1773 - 1828), industrialist and philanthropist
- MADOG ap GRUFFYDD (d. 1236), lord of Powys
- MADOG ap GWALLTER, friar, a religious poet
- MADOG ap LLYWELYN (fl. 1294), rebel
- MADOG ap MAREDUDD (d. 1160), king of Powys
- MADOG BENFRAS (fl. c. 1320-1360), poet
- MADOG DWYGRAIG (fl. c. 1370), poet
- MADOG FYCHAN ap MADOG ap GRUFFYDD (d. 1269), son and brother to the Princes of Powys Fadog
- MADRYN family Madryn, Llŷn
- MAELGWN ab OWAIN GWYNEDD (d. 1173), prince of Anglesey
- MAELGWN ap RHYS (c. 1170 - 1230), lord of Ceredigion
- MAELGWN ap RHYS (d. 1295), rebel of 1294
- MAELGWN GWYNEDD (d. c. 547), king of Gwynedd and monk
- MAELOG - see JOHNES, ARTHUR JAMES
- MAELOR of RHOS, Baron - see JONES, THOMAS WILLIAM
- MAIDOC Saint - see AIDAN, Saint
- MAINWARING, WILLIAM HENRY (1884 - 1971), Labour politician
- MAIR EIFION - see DAVIES, MARY
- MALKIN, BENJAMIN HEATH (1769 - 1842), antiquary and author
- MALO (fl. 6th century), saint
- MAM CYMRU - see KATHERYN of BERAIN
- MAM O NEDD - see COOMBE TENNANT, WINIFRED MARGARET
- MANNAY, JAMES (JIM) SAPOE JOHN (Ahmed Hassan Ismail) (1927 - 2012), historian and poet
- MANOETHWY - see JONES, OWEN
- MANSEL family Oxwich, Penrice, Margam abbey,
- MANSEL, BUSSY (1623 - 1699) Briton Ferry, parliamentary commander and Member of Parliament
- MANSEL, Sir ROBERT (1573 - 1656), admiral
- MANSEL, WILLIAM LORT (1753 - 1820), Master of Trinity, and bishop
- MANSELL, FRANCIS (1579 - 1665), principal of Jesus College, Oxford
- MANUEL, DAVID (1624? - 1726), poet
- MAP, WALTER (1140? - 1209?), archdeacon
- MARC, SIARL (1720 - 1795) Tŷ-mawr, Bryncroes, Llŷn, Calvinistic Methodist exhorter, and hymn-writer
- MARCH, 4th Earl of - see MORTIMER, ROGER de
- MARDY-JONES, THOMAS ISAAC (1879 - 1970), economist and politician
- MAREDUDD ab OWAIN ab EDWIN (d. 1072), king of Deheubarth