In a 5-3 decision,* the Court ruled in favor of Mapp. The majority opinion, written by Justice Clark, applied the exclusionary rule to the states. That rule requires courts to exclude from criminal trials evidence that was obtained in violation of the constitution’s ban on unreasonable searches and arrests. Justice Harlan wrote a dissenting opinion. The majority opinion was based on several earlier decisions that had begun the process of applying federal constitutional protections to state criminal justice systems.

In one of those earlier decisions, the Supreme Court had ruled that the states must be bound by the Fourth Amendment because its guarantees were part of the “due process of law” required of states by the 14th Amendment. That decision essentially required the Fourth Amendment’s provisions, which previously had only applied to the federal government, to apply to the states as well. The justices ruled that since the guarantees of the Fourth Amendment applied to both the federal and state governments, they should be enforced the same way in both federal and state courts. Evidence obtained unlawfully is not admissible in federal court, so it should not be admissible in state courts either. 

The justices reasoned that requiring states to obey to the exclusionary rule created “no war between the Constitution and common sense.” They responded to the argument that the exclusionary rule would make it possible for criminals to go free due to police error by pointing out that “the criminal goes free, if he must, but it is the law that sets him free.” The justices stated that the exclusionary rule was necessary to make state authorities abide by the requirements of the Fourth Amendment, for “nothing can destroy a government more quickly than its failure to observe its own laws.” Thus, the Court decided that “the exclusionary rule is an essential part of both the Fourth and 14th Amendments.”

In his dissent, Justice Harlan argued that the majority had confronted the wrong issue in its decision. Because Ms. Mapp was convicted under an Ohio statute criminalizing the possession of obscene material, Justice Harlan believed that the “new and pivotal issue” was whether this statute “is consistent with the rights of free thought and expression assured against state action by the 14th Amendment.” Thus, he concluded that the majority had ignored the principles of judicial restraint and stare decisis, and had “’reached out’” to consider the exclusionary rule issue. According to Justice Harlan, this was a First Amendment case and not an appropriate case for extending the Fourth Amendment’s exclusionary rule to the states.  He also concluded that it was wrong to impose the exclusionary rule, designed for the federal criminal process, on the states which, in his view, bore quite different responsibilities in this area of law.

*Justice Stewart wrote a separate opinion that did not address the issue of the exclusionary rule. He voted to reverse Mapp’s conviction solely on First Amendment grounds.