MacKinnon is the most practically consequential feminist theorist of the twentieth century, because she converted a theoretical position into an enforceable legal category.
Sexual Harassment of Working Women (1979) argued that unwanted sexual conduct at work is not a personal matter between individuals but sex discrimination under Title VII, since it imposes a condition of employment on women that is not imposed on men. Courts had rejected such claims; within a decade the Supreme Court had accepted the argument, and the category she constructed is now law in much of the world.
The underlying theory is the dominance approach, set against both the sameness and difference strands of feminist legal thought. Equality doctrine, she argues, asks whether women are similarly situated to men and therefore takes the male position as the unmarked standard against which claims are measured. The real question is not difference but hierarchy: who is on top. On this account sexuality is where the hierarchy is made and eroticized, so that consent is a poor test in conditions of unequal power.
The controversial application followed. With Andrea Dworkin she drafted ordinances defining pornography as a practice of sex discrimination and giving women harmed in its production or use a civil remedy — an approach that treated it as an act rather than as speech. Passed in Minneapolis and Indianapolis, it was struck down as unconstitutional, and it split the movement: anti-censorship feminists organized against it, and the resulting sex wars of the 1980s have never fully closed. Her later work took sexual violence in war to international tribunals, where the argument that rape can constitute genocide was accepted.