Chorpenning v. United States was a United States Supreme Court case that dealt with the issue of whether a contract between the United States and a private individual was valid. The case involved a contract between the United States and a man named Chorpenning, who had been hired to deliver mail between two military posts in California. Chorpenning had been paid for his services, but the United States later refused to pay him for additional services he had performed. Chorpenning sued the United States for breach of contract. The Supreme Court held that the contract between the United States and Chorpenning was valid and enforceable. The Court noted that the contract was made in good faith and that the United States had accepted the services provided by Chorpenning. The Court also noted that the United States had paid Chorpenning for his services, and that the United States had not provided any evidence that the contract was invalid. The Court concluded that the contract was valid and enforceable, and that Chorpenning was entitled to payment for the additional services he had provided.

In Chorpenning v. United States, the Supreme Court was tasked with determining whether a contract between the plaintiff and the government was validly entered into. The majority opinion held that it was not, as there had been no consideration given by either party to support its validity. Justice Field dissented from this conclusion on two grounds: first, he argued that an implied promise of payment could be inferred from past dealings between the parties; second, he noted that even if such a promise did not exist in fact or law at the time of contracting, it would still be binding upon both parties due to their subsequent conduct in executing and performing under it for several years prior to litigation being initiated. In sum, Justice Field believed that sufficient evidence existed to demonstrate mutual assent and consideration necessary for a valid contract between Chorpenning and the United States government.