Germain v. 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 a federal prison. The case arose when a prisoner, Germain, was held in a federal prison in the state of New York. The state court issued a writ of habeas corpus to the warden of the prison, ordering him to produce Germain in court. The warden refused to comply, arguing that the state court did not have the authority to issue the writ. The Supreme Court ultimately held that the state court did not have the authority to issue the writ of habeas corpus. The Court reasoned that the writ of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal prison system. 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. In conclusion, 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 power, and that the state court should not be allowed to interfere with the federal government's power to protect this right.

In Germain v. Mason, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving an alleged breach of contract between two citizens of different states. The majority opinion held that the state court did have jurisdiction and could proceed with its decision in favor of one party. However, Justice Field dissented from this opinion on the grounds that it violated Article III Section 2 Clause 1 of the United States Constitution which grants exclusive federal judicial power over cases where parties are citizens of different states or foreign countries. He argued that since both parties were citizens from different states, only a federal court should hear their dispute and not a state court as they lacked authority to do so under constitutional law. Furthermore, he noted that if such disputes were allowed to be heard by state courts then there would be no uniformity in decisions across all jurisdictions as each individual judge may interpret laws differently leading to conflicting results for similar cases tried in multiple courts at once.