Mayer et al. v. Hellman was a United States Supreme Court case that dealt with the issue of whether a state court could enforce a contract that was made in violation of a state statute. The case involved a dispute between two parties, Mayer and Hellman, over a contract that Mayer had made with Hellman to purchase a certain amount of stock. The contract was made in violation of a California statute that prohibited the sale of stock without a license. The Supreme Court held that the state court could not enforce the contract because it was made in violation of the state statute. The Court reasoned that the state statute was a valid exercise of the state's police power and that the state had the right to protect its citizens from fraud and other forms of financial harm. The Court also noted that the contract was not a valid contract because it was made in violation of the state statute. The Court's decision in Mayer et al. v. Hellman established that a state court cannot enforce a contract that is made in violation of a state statute. This decision has been cited in numerous cases since then and has been used to support the idea that a state has the right to protect its citizens from fraud and other forms of financial harm.

In Mayer et al. v. Hellman, the Supreme Court was asked to decide whether a California state court had jurisdiction over a dispute between two non-residents of the state concerning real estate located in California. The majority opinion held that since both parties were non-residents and neither party had any property or rights within the state, there was no basis for exercising jurisdiction by the California court. Justice Field dissented from this decision on grounds that it would be unfair to allow one party to escape liability simply because they are not residents of the same state as their adversary's property is located in. He argued that if such an interpretation of law were allowed then it could lead to injustice and fraud against those who have invested money into land situated in another jurisdiction without having any legal recourse available should disputes arise with other parties involved with said land investments.