Mackay v. Easton was a United States Supreme Court case that dealt with the issue of whether a contract between two parties was valid. The case involved a contract between Mackay and Easton, in which Mackay agreed to pay Easton a certain amount of money for the sale of a piece of property. Easton had previously agreed to sell the property to another party, but Mackay argued that the contract between him and Easton was still valid. The Supreme Court ultimately ruled in favor of Mackay, finding that the contract between the two parties was valid. The Court held that the contract was binding, even though Easton had already agreed to sell the property to another party. The Court reasoned that Easton had not yet completed the sale to the other party, and thus the contract between Mackay and Easton was still valid. The Court's decision in Mackay v. Easton established that a contract between two parties is still valid, even if one of the parties has already agreed to sell the property to another party. This ruling has been cited in numerous cases since, and has become an important precedent in contract law.

In the case of Mackay v. Easton, the Supreme Court was asked to decide whether a contract between two parties could be enforced when it had been made in violation of a state law. The majority opinion held that such contracts were not enforceable and that any rights or obligations created by them would have no legal effect. Justice Field dissented from this decision, arguing that while states may pass laws regulating certain types of contracts, they cannot invalidate those contracts altogether unless they are contrary to public policy or morals. He argued further that if courts refuse to enforce validly formed agreements simply because they violate some state statute, then individuals will be deprived of their right to make binding contractual arrangements with one another without fear of judicial interference.