Lyon v. Pollock 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 habeas corpus to a prisoner held in federal custody. The case arose when a prisoner, William Lyon, was held in federal custody in the state of Missouri. Lyon sought a writ of habeas corpus from the state court, which the court granted. The federal government then appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the writ of habeas corpus was a federal prerogative, and that the state court did not have the authority to interfere with the federal government's power to detain a prisoner. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect this right. The Court's decision in Lyon v. Pollock established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal custody. This decision has been cited in numerous subsequent cases, and has been used to support the idea that the federal government has the power to protect the fundamental right of habeas corpus.

In Lyon v. Pollock, the Supreme Court was tasked with deciding whether a state court had jurisdiction to hear a case involving an alleged breach of contract between two citizens of different states. The majority opinion held that the state court did not have jurisdiction because it lacked diversity among its parties and thus could not exercise federal judicial power under Article III of the Constitution. Justice Field dissented from this ruling, arguing that since both parties were citizens of different states at the time when they entered into their agreement, there existed sufficient diversity for federal courts to take up their dispute in accordance with Article III's requirements. He further argued that even if such diversity no longer existed by the time suit was brought before a state court, it should still be allowed as long as neither party objected or raised any objection prior to trial proceedings beginning in earnest.