The rising field of medical jurisprudence in common law from late eighteenth century has led to a rearrangement of authority and epistemic power between lay and expert witnesses, in favor of the latter. Although the law had long relied on testimony from members of the community to establish the legal fact of a person's sex, the legal procedure of fact-making started to rely instead on the opinions of doctors, surgeons, and medical practitioners. This article closely reads medical jurisprudence books, U.S. case law, and U.S. newspapers from the nineteenth century to describe this expansion of medical experts' authority to establish the legal fact of sex in vague cases. The article describes the spread of medico-legal technics of sex classification in three arenas of U.S. law: the law of marriage and divorce, cross-dressing, and defamation. The practice of legal sex classification was thus absorbed into medical expertise, and the meaning of sex in the law transformed from a socio-physical construct to a medical one. The mid-nineteenth-century decline of medical jurisprudence subsequently pushed the practice of sex classification outside the realm of law and into the jurisdiction of the medical profession, thus leaving sex classification mainly to doctors.