Leftwitch v. Lecanu was a case heard by the United States Supreme Court in 1866. The case involved a dispute between two parties over a contract for the sale of a steamboat. The plaintiff, Leftwitch, had contracted with the defendant, Lecanu, to purchase a steamboat for $2,000. Leftwitch paid the full amount, but Lecanu refused to deliver the boat. Leftwitch then sued Lecanu for breach of contract. The Supreme Court held that the contract was valid and enforceable. The Court found that the contract was supported by consideration, meaning that both parties had given something of value in exchange for the other's promise. The Court also found that the contract was not void for lack of mutuality, meaning that both parties had agreed to the same terms. The Court held that Lecanu was liable for breach of contract and ordered him to deliver the steamboat to Leftwitch. The Court also ordered Lecanu to pay damages to Leftwitch for the breach. This case established the principle that a contract is valid and enforceable if it is supported by consideration and mutuality.

In Leftwitch v. Lecanu, the Supreme Court was tasked with deciding whether a contract between two parties that had been made in Louisiana before it became part of the United States was still valid after Louisiana joined the Union. The majority opinion held that since Louisiana had become a state and adopted its own laws, any contracts made prior to this were no longer binding under those new laws. However, Justice Field dissented from this decision on the grounds that such an interpretation would be unfair to both parties involved in making these pre-statehood agreements as they could not have anticipated their contracts becoming invalidated by something outside of their control. He argued instead for upholding these agreements as long as they did not violate public policy or morals and noted that if one party chose to breach them then legal remedies should be available for damages incurred due to such actions.