Mackall v. Richards was a United States Supreme Court case that addressed 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 the petitioner, John Mackall, was arrested by federal officers in the District of Columbia and taken to the state of Maryland, where he was held in federal custody. The state of Maryland then sought to issue a writ of habeas corpus to Mackall, which the federal government opposed. 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 did not have the authority to interfere with the federal government's exercise of its power. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court could not interfere with the federal government's exercise of its power to protect 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 did not have the authority to interfere with the federal government's exercise of its power. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court could not interfere with the federal government's exercise of its power to protect that right.

In the case of Mackall v. Richards, Justice Harlan delivered a dissenting opinion in which he argued that the majority had failed to consider an important point: whether or not the plaintiff was entitled to recover damages for breach of contract. The majority held that since no consideration had been given by either party when entering into the agreement, there could be no recovery; however, Justice Harlan disagreed with this conclusion and argued that consideration should have been taken into account as it is essential for any valid contract. He further noted that even if there was no actual exchange of money or goods between parties at the time they entered into their agreement, both sides still provided something valuable - namely their promises - which constituted sufficient consideration under existing law. As such, he concluded that it would be unjust to deny recovery on these grounds alone and urged his colleagues to reconsider their decision accordingly.