Millett died of a heart attack while on a visit to Paris on Wednesday, according to a person with knowledge of the matter, who spoke on condition of anonymity because he was not authorised to speak for the family.
The publishing house that carried her books in French also confirmed the death but provided no details.
Sexual Politics was published in 1970, in the midst of feminism's so-called "second wave."
Millett's book was among the most talked-about works of its time and remains a founding text for cultural and gender studies programmes.
It chronicled millennia of legal, political and cultural exclusion and diminishment of women.
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