Roberts v. Reilly 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 compel a county auditor to issue a tax deed. The case was brought by the plaintiff, Roberts, who sought to compel the defendant, Reilly, to issue a tax deed for a piece of property that Roberts had purchased at a tax sale. The Supreme Court held that the state court did not have the authority to issue a writ of mandamus to compel the county auditor to issue a tax deed. The Court reasoned that the power to issue a tax deed was a power that was vested in the county auditor, and that the state court did not have the authority to interfere with the county auditor's exercise of that power. The Court further held that the state court could not issue a writ of mandamus to compel the county auditor to issue a tax deed because the county auditor was not a party to the action. In conclusion, the Supreme Court held that the state court did not have the authority to issue a writ of mandamus to compel the county auditor to issue a tax deed. The Court reasoned that the power to issue a tax deed was a power that was vested in the county auditor, and that the state court did not have the authority to interfere with the county auditor's exercise of that power.

Justice Field delivered the dissenting opinion in Roberts v. Reilly, arguing that the court should have affirmed the decision of the Supreme Court of California. He argued that under section 841 of the Revised Statutes, a state may tax all property within its jurisdiction and it is not necessary for Congress to pass legislation authorizing such taxation. Furthermore, he argued that since this case involved an assessment made by a state officer pursuant to a law passed by California's legislature and approved by its governor, there was no federal question presented as required for review in federal courts. Justice Field concluded his dissent with an argument against judicial activism: "The power thus assumed [by this court] has been exercised only when absolutely necessary; but here we are asked to assume it without any necessity whatever."