Gardner v. Brown was a United States Supreme Court case that addressed the issue of whether a state could tax the income of a non-resident who was employed by the state. The case was brought by a resident of the state of New York, who was employed by the state of California. The plaintiff argued that the state of California had no right to tax his income, as he was a non-resident of the state. The Supreme Court held that the state of California had the right to tax the income of the plaintiff, as he was employed by the state and was thus subject to the state's taxing authority. The Court reasoned that the state had a legitimate interest in taxing the income of its employees, as it was necessary to fund the state's operations. The Court also noted that the state had a legitimate interest in ensuring that its employees paid their fair share of taxes. The Court concluded that the state of California had the right to tax the income of the plaintiff, and that the plaintiff was not entitled to any special exemption from taxation.

Justice Field delivered the dissenting opinion in Gardner v. Brown, arguing that California's tax on mining companies was unconstitutional because it violated the Contract Clause of the United States Constitution. He argued that when a state passes a law which impairs an existing contract between two parties, it is in violation of Article I Section 10 of the US Constitution and thus invalid. In this case, he believed that California had done just that by passing its Mining Tax Act and imposing taxes on mining companies who had already entered into contracts with landowners for mineral rights prior to passage of said act. Justice Field further argued that since these contracts were made before any such laws existed, they should be considered valid under both federal and state law as well as protected from interference by either party or government entity. As such, he concluded that California's Mining Tax Act was unconstitutional and should be struck down accordingly.