Mary O'Neill may refer to:
- Marykate O'Neil (born 1968), American indiepop singer-songwriter
- Mary O'Neill (Canadian politician) (born 1941), Canadian member of the Legislative Assembly of Alberta
- Mary Devenport O'Neill (1879–1967), American poet
- Mary O'Neill (fencer) (born 1965), American Olympic fencer
- Mary-Anne O'Neill (born 1955), Australian politician