Also known as: mistake of law · mistake of law defense · ignorance of the law is no excuse · ignorance of law is no defense · ignorantia juris non excusat
Written by attorneys · grounded in primary & secondary sources — see below
A principle of law providing that a defendant's erroneous belief that conduct is not prohibited by statute does not excuse liability. The defense is unavailable even when the mistake is reasonable unless the mistake negates a required mental state or the defendant reasonably relied on an official statement of law later determined to be erroneous.
Miranda Morales removed a neighbor's lawn equipment believing a local ordinance gave her the right to clear items left on a shared driveway. The ordinance did not apply to the equipment. Her mistaken view of the law did not relieve her of liability for trespass to chattels.
Meredith Maxwell divided cash deposits to avoid bank reporting thresholds while believing the practice was lawful. Prosecutors proved she knew the reporting rules existed. The mistake about legality supplied no defense to the structuring charge.
Select any source to read its text and confirm it supports the definition.
Michelle Murphy refused to file returns after concluding wages were not taxable income. She had received official IRS publications stating the opposite. The mistaken legal conclusion did not negate the willfulness element required for conviction.
Maria Morales kept a rifle after modifying it to fire automatically while believing the modification did not trigger registration requirements. Evidence showed she knew the weapon's features. The mistake of law provided no defense to the possession charge.
Marco Marquez sold guns after concluding his occasional sales fell outside licensing statutes. He had reviewed the statutes but misinterpreted their reach. The erroneous legal interpretation did not excuse the unlicensed dealing offense.
Melissa Mills posted violent messages about a coworker while believing the statements were protected speech. The messages caused reasonable fear. The mistaken view of First Amendment coverage supplied no defense to the threat charge.
A mistake of law supplies a defense when it negates a required mental state or when the defendant reasonably relied on an official statement of law later found erroneous. Reliance on a private blog or internal company materials does not qualify.
Yes when the pamphlet comes from the agency charged with administering the law and the reliance is reasonable. The defense fails if the statement is unofficial or the defendant ignores explicit limitations in the official guidance.
Yes when the enactment was not published or reasonably made available before the conduct. The defense requires proof by a preponderance that the defendant lacked actual knowledge and the government failed to provide reasonable access.
No. Internal materials from a private employer are not official statements of law. The defense arises only when the statute itself makes the mistaken belief exculpatory or when reliance is on an official public source.
…188, 191-193 (CA1 1985), we granted certiorari, 493 U. S. 1068 (1990). II The general rule that ignorance of the law or a mistake of law is no defense to criminal prosecution is deeply rooted in the American legal system. See, e. g. , United States v. Smith , 5 Wheat. 153, 182 (1820) (Livingston, J., dissenting); Barlow v. United…