Probably her most well known works are on existentialism, conformity, and authenticity that challenged social norms and conventions and was influenced by and influenced in turn the work of Jean-Paul Sartre.

The most iconic legacy and the one I hear the most would be her existentialist works primarily her assertion that "Woman is the Other" where she applied the existentialist idea that Existence precedes Essence, and the existentialist Hegel's concept of the Other to the female gender. The Hegelist Other, is the aberration and thereby the inferior, for example say I were to state that I have a pet rat, you would assume that it was a perfectly average specimen of a rat because I did not say anything to indicate that it was normal or not. Now if I had an albino rat, I would probably state that even if it were not directly relevant because while many rats are albino, they are an aberration and the fact that they are albino changes the nature of my discussing them with you. The concept of the Other applies the same process to human social norms, one of my favourite analogies that I remember reading was that a newspaper might say that X is a comedian when referring to a straight comedian but feel the need to describe them as gay comedian if they were gay, the same for an African-American US president or a female astronaut although none of those prefixes really give the hypothetical reader more information than they would get from a male astronaut, or a white US president or a straight comedian. But, they are important because we then proceed to define the individuals in question not just by the information of their actions, words or accomplishments, but within the confines of their alternate identity, where they deviate from the social norm.

DeBeauvoir reasoned that since men were neutral, positive and the norm (until you know the gender of something or someone it is almost always a he), than women were the Other as Hegel would say. But she further argued that although humans were born male or female, no one was born a Woman, because that entailed the learning of various gender constructs from the society or culture a person inhabited, and that by breaking out of that construct could graduate from immanence to transcendence and be truly free to take personal and social responsibility as fully realized person.

Probably her largest individual contribution was in changing the way contemporary feminists viewed gender itself and fueling further debate of concepts like femininity and sexuality, but she was actually hesitant to call herself a feminist despite the fact that many of her views were congruent with feminism. She also helped to push for increased womens rights, equality, and legalization of abortion in France.