Ma-Mi
- Jules François Mabille (1831–1904), French malacologist
- John Macadam, Scottish-born Australian botanist
- John M. MacDougal (born 1954), American botanist
- William MacGillivray (1796–1852), Scottish naturalist
- Marcello Malpighi (1628–1694), Italian anatomist and biologist
- Ramon Margalef (1919–2004), Spanish-Catalan biologist and ecologist
- Leo Margolis (1927–1997), Canadian fisheries parasitologist
- Lynn Margulis (born 1938), American microbiologist
- Alberto della Marmora (1789–1863), Italian naturalist
- Othniel Charles Marsh (1831–1899), paleontology
- Barry Marshall (born 1951), Australian physician and microbiologist, winner of the 2005 Nobel Prize in Physiology or Medicine for his discovery that most stomach ulcers are caused by a strain of bacteria
- Fermín Martín Piera (1954–2001), Spanish botanist
- Carl Friedrich Philipp von Martius (1794–1868), German botanist
- John Martyn (1699–1768), English botanist
- Thomas Martyn (1735–1825), English botanist, entomologist and conchologist
- John Marwick (1891–1978), New Zealand palaeontologist and geologist
- Francis Masson (1741 – c. 1805), Scottish botanist
- Gregory Mathews (1876–1949), Australian ornithologist
- Paul Matschie (1861–1926), German zoologist
- William Diller Matthew (1871–1930), American paleontologist
- Polly Matzinger, American immunologist
- Carl Maximowicz (1827–1891), Russian botanist
- Harold Maxwell-Lefroy (1877–1925), English entomologist
- Robert May (born 1936), biologist, physicist, mathematician, President of Royal Society of London 2000–2005
- Ernst Mayr (1904–2005), evolutionary biologist
- Barbara McClintock (1902–1992), American biologist, winner of a Nobel Prize for her work on the transposon, or "jumping gene"
- James V. McConnell (1925–1990), American biological psychologist
- Mark McMenamin (born 1958), American paleontologist
- Bruce McEwen Neuroendocrinologist and stress hormone expert
- Edmund Meade-Waldo (1855–1934), English ornithologist
- Ilya Ilyich Mechnikov (1845–1916), Russian microbiologist, best known for his work on the immune system and phagocytosis, received the Nobel Prize in Physiology or Medicine in 1908
- Johann Wilhelm Meigen (1764–1845), German entomologist
- Gregor Mendel (1822–1884), Czech-Austrian monk who is often called the "father of genetics" for his study of the inheritance of traits in pea plants
- Edouard Menetries (1802–1861), French entomologist
- Maud Leonora Menten, biologist
- Archibald Menzies (1754–1852), Scottish naturalist
- Clinton Hart Merriam (1855–1942), American zoologist and ornithologist
- John C. Merriam (1869–1945), American biologist
- Franz Meyen (1804–1840), German botanist
- Rodolphe Meyer de Schauensee (1901–1984), American ornithologist
- Otto Fritz Meyerhof (1884–1951), German/American physician and biochemist, winner of the 1922 Nobel Prize in Physiology or Medicine for his research on muscles
- Leonor Michaelis (1875–1949), German biologist
- André Michaux (1746–1802), French botanist
- Aleksandr Fyodorovich Middendorf (1815–1894), Russian zoologist
- Nicholai Miklukho-Maklai (1846–1888), Russian marine biologist and anthropologist
- Gerrit Smith Miller, Jr. (1869–1956), American zoologist.
- Jacques Miller (born 1931), Australian immunologist.
- John Frederick Miller (1759–1796), English illustrator (primarily of botany)
- Kenneth R. Miller (1948-), American evolutionary biologist.
- Philip Miller (1691–1771), Scottish botanist (abbr. in botany: Mill.)
- Alphonse Milne-Edwards (1835–1900), French zoologist
- Henri Milne-Edwards (1800–1885), French zoologist
- George Jackson Mivart (1827–1900), English biologist
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