The Fourth Amendment to the U.S. Constitution provides: "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated...." Until the early 20th century, Americans' only legal remedy in cases where law enforcement officers violated the Fourth Amendment was a private lawsuit against the officers involved, either in trespass to recover damages, or in replevin to recover their seized goods or property.

The tendency of those who execute the criminal laws of the country to obtain a conviction using unlawful seizures ... should find no sanction in the judgments of the courts, which are charged at all times with the support of the Constitution, and to which people of all conditions have a right to appeal for the maintenance of such rights.Over the next several decades, the Court generally held that this "exclusionary rule" only applied to cases in which federal law enforcement officers, not state officers, were involved in illegal searches and seizures.

[5] Dollree "Dolly" Mapp was a young woman in Cleveland, Ohio who in the mid-1950s became involved in the illegal gambling operations of mobster and racketeer Shondor Birns, who dominated organized crime in the city.

Mapp was found guilty at trial of "knowingly having had in her possession and under her control certain lewd and lascivious books, pictures, and photographs in violation of 2905.34 of Ohio's Revised Code," and sentenced to one to seven years in prison.

The Court observed that of the 30 U.S. states that had rejected the exclusionary rule at the time of Wolf v. Colorado in 1949, more than half had adopted at least a partial form of it in the intervening 12 years.

"[13] Justice Clark stated that without the exclusionary rule, the Fourth Amendment's protections against unreasonable searches and seizures would be merely "a form of words" that would be "valueless and undeserving of mention in a perpetual charter of inestimable human liberties.

[11] In a frequently quoted passage, the Court wrote: [O]ur holding that the exclusionary rule is an essential part of both the Fourth and Fourteenth Amendments is not only the logical dictate of prior cases, but it also makes very good sense.

Thus, the State, by admitting evidence unlawfully seized, serves to encourage disobedience to the Federal Constitution which it is bound to uphold.Clark concluded the Court's opinion by reiterating how the "ignoble shortcut" around the Fourth Amendment that Wolf had left open to state law enforcement officers had "tend[ed] to destroy the entire system of constitutional restraints on which the liberties of the people rest," and subsequently reversed the Supreme Court of Ohio's judgment against Dolly Mapp.

Harlan wrote that the Court should not have reached the Fourth Amendment issues in the case, because Mapp's conviction and the subsequent arguments at her appeals had focused on the legality of Ohio's anti-pornography laws, and not on the police officers’ warrantless search of her home.