In the United States Supreme Court case of The United States v. Pearson B. Reading, the court was asked to decide whether a person who had been convicted under an act of Congress could be pardoned by the President without being discharged from his sentence. Reading had been convicted in 1851 for violating a law that prohibited trading with Native Americans on their reservations and he received a two-year prison sentence as punishment. In 1853, President Franklin Pierce issued him a pardon but refused to discharge him from his sentence until after it expired naturally in 1855. The question before the court was whether this action violated Article II Section 2 Clause 1 of the Constitution which states that “the president shall have power to grant reprieves and pardons for offenses against the United States” or if it fell within another clause which gives presidents authority over executive functions such as issuing regulations concerning federal prisons and prisoners therein? After considering both sides' arguments, Chief Justice Taney delivered an opinion stating that while presidents do indeed possess broad powers when it comes to granting clemency they cannot use those powers so as to modify sentences imposed by acts of Congress; thus, Pierce's refusal to discharge Reading from his sentence was unconstitutional and voidable at law.

In the case of United States v. Pearson B. Reading, the Supreme Court was asked to decide whether a defendant could be convicted for an act that had been declared unconstitutional by Congress after it occurred but before his trial began. The majority opinion held that since the act was still in effect when Reading committed it, he could be found guilty and sentenced accordingly. However, Justice McLean dissented from this decision on two grounds: firstly, because he believed that punishing someone for violating a law which has subsequently been repealed would violate their right to due process; and secondly, because such punishment would amount to ex post facto legislation - something prohibited by both state and federal constitutions at the time of this ruling. He argued instead that if Congress had intended to punish individuals who violated laws prior to their repeal then they should have explicitly stated so in those laws themselves or provided some other form of relief for those affected by them retroactively.