Ex parte Mason 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 petitioner, Mason, was a prisoner in the custody of the United States Marshal for the District of Columbia. He had been convicted of a crime in the District of Columbia and was serving a sentence in the federal penitentiary. Mason sought a writ of habeas corpus from the Supreme Court of the District of Columbia, claiming that his conviction was unconstitutional. The Supreme Court of the District of Columbia granted the writ, and the United States Marshal brought Mason before the court. The United States Attorney for the District of Columbia argued that the state court did not have the authority to issue the writ, as the prisoner was in federal custody. The Supreme Court of the District of Columbia disagreed, ruling that the state court had the authority to issue the writ. The United States appealed the decision to the United States Supreme Court. The Supreme Court affirmed the decision of the Supreme Court of the District of Columbia, ruling that the state court had the authority to issue the writ of habeas corpus. The Court held that the writ of habeas corpus was a fundamental right, and that the state court had the authority to issue the writ even when the prisoner was in federal custody. The Court also held that the writ of habeas corpus was a necessary part of the due process of law, and that the state court had the authority to issue the writ in order to protect the rights of the prisoner.

In Ex Parte Mason, the Supreme Court was tasked with determining whether a writ of habeas corpus should be issued to release an individual from prison. The majority opinion held that the writ should not be granted because it had been denied by lower courts and there were no new facts presented in this case. Justice Field dissented, arguing that the petitioner's rights under the Constitution had been violated due to his confinement without being charged or tried for any crime. He argued that since he was never convicted of a crime, he could not have waived his right to challenge his imprisonment through a petition for habeas corpus. Furthermore, Justice Field noted that even if all other requirements were met for denying such petitions - including lack of jurisdiction and failure to exhaust state remedies - those conditions did not apply here as none existed in this particular case. As such, he concluded that granting relief would be appropriate given these circumstances and thus voted against denying the writ of habeas corpus sought by petitioner Mason