Commissioners of Johnson County v. Thayer was a United States Supreme Court case that dealt with the issue of taxation. The case involved a dispute between the Commissioners of Johnson County, Kansas and a man named Thayer. Thayer had purchased a piece of land in Johnson County and had paid the taxes due on the land. However, the Commissioners of Johnson County had assessed additional taxes on the land, which Thayer refused to pay. Thayer argued that the additional taxes were unconstitutional and that he should not be required to pay them. The Supreme Court ultimately sided with Thayer, ruling that the additional taxes were unconstitutional. The Court held that the additional taxes were a violation of the Fourteenth Amendment, which prohibits states from denying any person the equal protection of the laws. The Court also held that the additional taxes were a violation of the Due Process Clause of the Fourteenth Amendment, which prohibits states from depriving any person of life, liberty, or property without due process of law. The Court's decision in this case established an important precedent for taxation in the United States. It established that states cannot impose taxes that are not based on a rational basis or that are not applied equally to all persons. This decision has been cited in numerous subsequent cases involving taxation and the Fourteenth Amendment.

In Commissioners of Johnson County v. Thayer, the Supreme Court was tasked with determining whether a county had the authority to levy taxes on railroad companies for their property within its boundaries. The majority opinion held that counties did not have such authority and that only state legislatures could impose such taxation. Justice Field dissented from this opinion, arguing that states were granted certain powers by the Constitution which included taxing private property in order to fund public services like roads and bridges; however, he argued that these same powers should be extended to local governments as well so they can provide essential services without relying solely on state funding or donations from citizens. He further noted that allowing counties to tax railroads would help ensure fairness since it would prevent them from avoiding paying taxes simply by moving across county lines.