In re Mills is a Supreme Court case from 1891 that dealt with the issue of whether a state court could issue a writ of habeas corpus to a prisoner who was being held in a federal prison. The petitioner, Mills, was a prisoner in a federal prison in the state of New York. He had been convicted of a federal crime and was serving his sentence in the federal prison. He then filed a petition for a writ of habeas corpus in the state court, arguing that he was being unlawfully detained in the federal prison. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner 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 power to interfere with the federal government's authority to imprison its citizens. The Court also noted that the writ of habeas corpus was a remedy for state prisoners, not federal prisoners. The Court's decision in In re Mills established that state courts do not have the authority to issue writs of habeas corpus to federal prisoners. This decision has been cited in numerous subsequent cases and has become an important precedent in the area of federal habeas corpus law.

In the dissenting opinion of In re Mills, Petitioner, Justice Black argued that the petitioner was denied due process when he was convicted without a jury trial. He noted that under the Sixth Amendment to the Constitution, all criminal defendants have a right to be tried by an impartial jury and that this right should not be taken away from them unless they waive it in writing. Furthermore, Justice Black stated that even if there had been evidence presented at trial which showed beyond a reasonable doubt that Mills committed murder or manslaughter (the charges for which he was convicted), his conviction would still violate his constitutional rights because he did not receive a fair hearing before being found guilty. Finally, Justice Black concluded by stating that while states may impose certain restrictions on their citizens' rights as long as those restrictions are consistent with due process of law and do not deny any person's fundamental liberties; here such safeguards were absent and thus violated Mills’s constitutional rights.