Stewart v. Masterson 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 Stewart, was held in federal custody in the state of New York. Stewart sought a writ of habeas corpus from the state court, which was denied by the court on the grounds that it lacked jurisdiction over federal prisoners. Stewart then appealed 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 lacked the power to interfere with the federal government's authority to detain prisoners. 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 exercise of that 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 federal custody. The Court reasoned that the writ of habeas corpus was a federal prerogative, and that the state court lacked the power to interfere with the federal government's authority to detain prisoners.

Justice Field delivered the dissenting opinion in Stewart v. Masterson, arguing that the majority had erred by failing to consider a prior decision of the Supreme Court of California which held that an agreement between two parties could not be enforced if it was made with knowledge of a third party's rights. He argued that this precedent should have been considered and applied in this case as well, since there were facts indicating that both parties knew or should have known about the third party's right to use part of the land for mining purposes when they entered into their agreement. Justice Field further noted that even though such agreements are generally enforceable under state law, they cannot be enforced if one or both parties had actual knowledge at the time of entering into them regarding any existing rights belonging to another person or entity. Therefore, he concluded, because there was evidence suggesting such knowledge on behalf of either party here, enforcement would violate public policy and thus must be denied.