Manufacturing Company v. Trainer was a case heard by the United States Supreme Court in 1879. The case involved a dispute between a manufacturing company and a former employee, Trainer. Trainer had been employed by the company for several years and had been promised a bonus if he stayed with the company for a certain period of time. When Trainer left the company before the end of the period, the company refused to pay him the bonus. Trainer sued the company for breach of contract. The Supreme Court held that the company was liable for breach of contract. The Court found that the company had made a valid promise to Trainer and that Trainer had relied on that promise in deciding to stay with the company. The Court also found that the company had not provided any evidence that Trainer had not fulfilled his part of the agreement. Therefore, the Court held that the company was liable for breach of contract and ordered it to pay Trainer the bonus he was owed.

In Manufacturing Company v. Trainer, the Supreme Court was tasked with deciding whether a contract between two parties could be enforced when it had been made in violation of a state statute. The majority opinion held that the contract should not be enforced because it violated public policy and thus was voidable at either party's request. Justice Field dissented from this decision, arguing that contracts are generally enforceable unless they violate some fundamental principle of justice or morality or conflict with an express provision of law. He argued that since there was no evidence to suggest any such violations here, the contract should have been enforced as written by both parties regardless of its violation of state statute.