St. Clair v. Cox was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of mandamus to a federal court. The case arose when the plaintiff, St. Clair, sought to have the defendant, Cox, removed from his position as a federal judge. St. Clair argued that Cox had acted in a manner that was prejudicial to the interests of the United States and that he should be removed from office. The state court issued a writ of mandamus ordering Cox to vacate his office. The Supreme Court held that the state court did not have the authority to issue a writ of mandamus to a federal court. The Court reasoned that the writ of mandamus was a form of judicial review, and that the power to review the actions of a federal court was reserved to the Supreme Court. The Court further held that the state court lacked the authority to issue a writ of mandamus to a federal court, and that the writ issued by the state court was therefore invalid. The decision in St. Clair v. Cox established the principle that state courts do not have the authority to issue writs of mandamus to federal courts. This decision has been cited in numerous subsequent cases, and it remains an important precedent in the area of federal court jurisdiction.

Justice Field delivered the dissenting opinion in St. Clair v. Cox, arguing that the majority had misconstrued the meaning of a state statute and failed to consider its legislative history. He argued that under Kentucky law, which was applicable in this case, an executor or administrator could not be held liable for debts incurred by their predecessor unless they were specifically named as such in a will or other legal document. Furthermore, he noted that even if there had been some ambiguity about whether an executor or administrator was responsible for prior debt obligations, it would have been up to the legislature—not courts—to resolve any confusion surrounding statutory language. In conclusion Justice Field asserted that since no evidence existed indicating otherwise, St Clair should not be held accountable for his predecessor's debt and thus should prevail on appeal