Mitchell v. St. Maxent's Lessee was a case heard by the United States Supreme Court in 1866. The case involved a dispute between two parties over the ownership of a tract of land in Louisiana. The plaintiff, Mitchell, claimed that he had purchased the land from the defendant, St. Maxent's Lessee, in 1845. St. Maxent's Lessee argued that the sale was invalid because it had not been approved by the Louisiana legislature. The Supreme Court ruled in favor of Mitchell, finding that the sale was valid and that Mitchell was the rightful owner of the land. The Court held that the Louisiana legislature had no authority to invalidate the sale, as it had not been approved by the legislature. The Court also held that the sale was valid under the Louisiana Civil Code, which provided that a sale of land was valid if it was made in good faith and with consideration. The Court's decision in Mitchell v. St. Maxent's Lessee established that the Louisiana legislature could not invalidate a sale of land that had been made in good faith and with consideration. The decision also established that the Louisiana Civil Code was the governing law in such matters. This decision has been cited in numerous subsequent cases involving the validity of land sales in Louisiana.

In Mitchell v. St. Maxent's Lessee, the Supreme Court was tasked with determining whether a state court had jurisdiction to hear an appeal from a federal district court in Louisiana when the original case involved land that had been granted by Congress under the Swamp Land Act of 1850. The majority opinion held that such appeals were not allowed and thus affirmed the decision of the lower courts denying jurisdiction to hear it. However, Justice Field dissented on this issue, arguing that since there was no express prohibition against such appeals in either statute or constitutional law, then they should be allowed as part of due process rights for litigants who are seeking justice through their legal system. He further argued that if Congress intended to deny these types of appeals then it would have done so explicitly rather than leaving them open-ended and subject to interpretation by individual states or judges within those states. Ultimately he concluded that allowing these types of appeals is necessary for ensuring fairness and equal protection under law which is essential for any functioning judicial system