Warnock v. Davis 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 mandamus to a county court. The case arose when the plaintiff, Warnock, sought to have the county court issue a writ of mandamus to the defendant, Davis, who was the county clerk. Warnock argued that the county court had the authority to issue the writ, while Davis argued that the state court had exclusive jurisdiction over the matter. The Supreme Court ultimately sided with Warnock, ruling that the county court had the authority to issue the writ of mandamus. The Court reasoned that the state court had no jurisdiction over the matter, as the writ of mandamus was a matter of county court jurisdiction. The Court further noted that the state court had no authority to interfere with the county court's decision. As such, the Court held that the county court had the authority to issue the writ of mandamus.

In the case of Warnock v. Davis, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a non-resident plaintiff against a resident defendant in which the cause of action arose outside of the state. The majority opinion held that such jurisdiction did not exist and reversed the judgment below. Justice Field dissented from this decision, arguing that it would be unjust for states to deny their courts authority over cases involving parties who are both citizens or subjects of other countries when those same courts have long exercised such authority in similar circumstances with respect to domestic litigants. He further argued that there is no constitutional provision prohibiting states from exercising jurisdiction in these matters and therefore they should be allowed to do so as part of their sovereign power under our federal system. In conclusion, he concluded that if Congress has not expressly prohibited it then each state should have discretion on how far its judicial powers extend within its own borders.