| Carex lurida Wahlenb., 1803 |
| Carex mackenziana Weath., 1936 |
| Carex macloviana d'Urv., 1825 |
| Carex macloviana subsp. subfusca (W. Boott) T. Koyama, 1989 |
| Carex macloviana var. subfusca (W. Boott) Kük., 1909 |
| Carex macrosperma Mack., 1909 |
| Carex madrensis L.H. Bailey, 1898 |
| Carex marcida Boott, 1839 |
| Carex marianensis Stacey, 1937 |
| Carex mcvaughii Reznicek, 1982 |
| Carex meadii Dewey, 1842 |
| Carex melanosperma Liebm., 1850 |
| Carex mesophila Reznicek & S. González, 2021 |
| Carex mexicana J. Presl & C. Presl, 1828 |
| Carex michoacana Reznicek, Hipp & S. González, 2007 |
| Carex microptera Mack., 1909 |
| Carex microrhyncha Mack., 1913 |
| Carex monticola Boeckeler, 1881 |
| Carex muhlenbergii Kunth ex Boott, 1862 |
| Carex muhlenbergii var. xalapensis (Kunth) Britton, 1894 |