Mary O'Neill

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