A basic but primary theme of Millett's book is her identification of sexual politics as a key societal issue. Though several seminal second-wave feminist works had come out before the publication of Sexual Politics in 1970—notably Simone de Beauvoir's The Second Sex in 1949 and Betty Friedan's The Feminine Mystique in 1963—Millett's book was nevertheless groundbreaking for its challenge to unjust and fixed gender roles, and it contributed to the definition of the burgeoning feminist movement. Much of what Millett asserted nearly fifty years ago is now widely accepted, but her book maintains the power to jar the present-day reader and show that controversy still surrounds feminist issues today.

In the book's title, "politics" refers to a patriarchal gender dynamic as it has existed throughout history and continues to exist: men exercise power over women. Much of Millett's analysis of the nature of patriarchy may seem evident today, but in 1970, when her book was published, this was not the case. As a trailblazer of radical second-wave feminism, Millett establishes principles by which the arbitrary nature of men's dominance is made clear—as is the analogy between it and other forms of oppression, such as that involving race. The existence of this patriarchal dynamic represents a kind of internal colonization and is perpetuated by a process of "socialization" in which both men and women are conditioned through the norms of their upbringing to regard the situation as natural and inevitable.

Millett identifies the first wave of feminism as occurring between 1830 and 1930. In other words, the second-wave movement of which she was a part was not unprecedented: the initial feminist phase was marked by a counterrevolution both in the policies of governments (chiefly the Soviet Union and Nazi Germany) and in ideology. Of particular interest is Millett's emphasis upon Freudian theory and the degree to which it attempts to uphold the patriarchal dynamic.

Millett devotes a large part of her elucidation of the sexual power dynamic to its instances in literature, with special emphasis on D. H. Lawrence, Henry Miller, Norman Mailer, and Jean Genet. Her analysis reveals the degree to which mainstream literature reflects not only the obvious dominance of men over women but also, at times, what can only be termed misogyny—even by authors generally considered to be progressives in other political arenas. An important point Millett delves into is the replication of male–female forms of dominance within the gay subculture in Jean Genet's writings.