Oneale v. Long was a case heard by the United States Supreme Court in 1807. The dispute arose when Thomas Oneale, an administrator of the estate of William Smith, sued John Long for failing to pay a debt owed to Smith's estate. In his defense, Long argued that he had already paid off the debt with two promissory notes given to Smith prior to his death and thus should not be held liable for any further payment. The Supreme Court ultimately sided with Oneale and ruled that although the promissory notes were valid instruments of payment at common law, they did not satisfy all debts due from one person to another under Virginia law because they lacked consideration or mutuality between parties; therefore, it was necessary for Long still make full payment on the original debt despite having issued two promissory notes beforehand.

In the case of Oneale v. Long, Chief Justice John Marshall delivered a dissenting opinion in which he argued that the court should not have dismissed the plaintiff's appeal for lack of jurisdiction. He asserted that although it was true that there had been no formal judgment entered by a state court, this did not mean that the Supreme Court could not hear an appeal from such proceedings. Instead, he argued, if there were sufficient evidence to show that a decision had been made and accepted by both parties as final then it should be considered binding on them and thus subject to review by the Supreme Court. Furthermore, Marshall noted how important it was for federal courts to protect citizens' rights against state infringement; therefore they must be able to hear appeals even when judgments are rendered without formalities or technicalities being observed.