Wadsworth v. Supervisors was a United States Supreme Court case that addressed the issue of whether a state could tax the income of a federal employee. The case was brought by William Wadsworth, a federal employee who was being taxed by the state of New York on his income. Wadsworth argued that the state was violating the Supremacy Clause of the United States Constitution, which states that federal law is the supreme law of the land. The Supreme Court agreed with Wadsworth, ruling that the state of New York was in violation of the Supremacy Clause. The Court held that the state was not allowed to tax the income of a federal employee, as this would be in direct conflict with federal law. The Court also held that the state was not allowed to tax the income of a federal employee in a way that was different from the way it taxed the income of other citizens. The Court's decision in Wadsworth v. Supervisors established an important precedent in the area of federalism. It established that states are not allowed to tax the income of federal employees in a way that is different from the way they tax the income of other citizens. This decision has been cited in numerous cases since then, and it has been used to protect the rights of federal employees.

In the case of Wadsworth v. Supervisors, the Supreme Court was asked to decide whether a county in New York had authority to levy taxes on certain lands owned by the plaintiff, William Wadsworth. The majority opinion held that it did not have such power and reversed an earlier decision from a lower court. Justice Field dissented from this ruling and argued that under New York law, counties were authorized to tax any land within their boundaries regardless of ownership or use. He further noted that if there was any ambiguity in state law regarding taxation powers then it should be interpreted liberally so as not to interfere with local government's ability to raise revenue for public purposes. In conclusion, he believed that since no clear prohibition existed against taxing these particular lands then they could be subject to taxation by the county without violating constitutional rights or principles of fairness.