McCoy v. Nelson 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 who was being held in a federal prison. The case arose when a prisoner, McCoy, was held in a federal prison in the state of Missouri. McCoy sought a writ of habeas corpus from the state court, claiming that he was being held in violation of his constitutional rights. The state court granted the writ, and the federal government 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 a federal prison. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a prisoner's detention, and not to challenge the conditions of the prisoner's confinement. The Court's decision in McCoy v. Nelson established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. The decision also clarified the scope of the writ of habeas corpus, and established that it could only be used to challenge the legality of a prisoner's detention, and not to challenge the conditions of the prisoner's confinement.

Justice Field delivered the dissenting opinion in McCoy v. Nelson, arguing that the majority's decision was wrongfully based on a misinterpretation of state law. He argued that under Missouri law, a contract between two parties could not be enforced if it had been entered into without consideration and with no intention to perform its terms. The majority had held that because there was an agreement between the parties for McCoy to pay Nelson $2,000 for his services as an agent in selling certain real estate owned by McCoy, this constituted sufficient consideration and thus rendered their contract enforceable. However Justice Field disagreed with this interpretation of Missouri law; he believed that since neither party intended to actually carry out their agreement at the time they made it (as evidenced by both sides' failure to take any action towards fulfilling its terms), then there was no valid consideration present and therefore no binding contract existed between them.