Mayor v. Sheffield was a United States Supreme Court case that dealt with the issue of whether a municipality had the power to impose a tax on the transfer of real estate. The case was brought by the Mayor of the City of New York, who argued that the city had the authority to impose a tax on the transfer of real estate within its jurisdiction. The Supreme Court held that the city did not have the power to impose such a tax, as it was not authorized by the state legislature. The Court reasoned that the power to tax was a power reserved to the state, and that the city could not exercise such a power without the express authorization of the state. The Court also held that the tax was not a valid exercise of the city's police power, as it was not necessary to protect the public health, safety, or welfare. The Court concluded that the tax was invalid and could not be enforced.

In Mayor v. Sheffield, the Supreme Court of the United States was tasked with determining whether a state law that required all children to attend public school violated the Fourteenth Amendment's Equal Protection Clause. The majority opinion held that it did not violate this clause because there was no discrimination based on race or class and thus no violation of equal protection rights. However, in his dissenting opinion Justice Field argued that while there may have been no intentional discrimination against any particular group, the effect of such a law would be to deny some individuals their right to choose which type of education they receive for their children. He further argued that if states are allowed to pass laws like these without violating equal protection rights then those same states could also pass laws denying certain groups access to other fundamental rights such as voting or owning property - something he believed would be unconstitutional under both federal and state constitutions.