@Yalow,
There's at least eight:
1. Mary Mitchell Holloway Wilhite (1831–1892), American physician, first female medical graduate from Indiana
2. Mary Mitchell Birchall (1840–1898), American first female recipient of bachelor's degree in New England
3. Mary Mitchell Slessor (1848–1915), Scottish Presbyterian missionary to Nigeria
4. Mary Mitchell (athlete) (1912–2007), New Zealand track and field champion
5. Mary Mitchell (landscape architect) (1923–1988), English co-author of 1988's The Pattern of Landscape
6. Mary Mitchell O'Connor (born 1959), Irish Fine Gael politician and cabinet minister
7. Mary Mitchell, American columnist for Chicago Sun-Times, writing since 1990s
8. Mary Jo Mitchell, American sports journalist since 2000s, a/k/a Mary Jo Perino and Mary Jo Ford