In a sweeping rebuke of the National Rifle Association, the nation’s most prominent gun rights group, a Manhattan jury ruled on Friday that its leaders had engaged in a yearslong pattern of financial misconduct and corruption.

The jury, after a week of deliberations, found that the group’s former leader, Wayne LaPierre, had used N.R.A. funds to pay for personal expenses, including vacations, luxury flights for his relatives and yacht rides, and that two other top executives had failed in their duties to the nonprofit organization.

The case, brought by New York’s attorney general, Letitia James, touched the uppermost echelons of the gun-rights group. In addition to Mr. LaPierre, the defendants included the group’s former treasurer, Wilson Phillips, and its general counsel, John Frazer, who still works for the group. The N.R.A. itself was also a defendant, and was found to have ignored whistle-blower complaints and submitted false filings to the state.

The N.R.A., founded after the Civil War to promote marksmanship, has been one of the most powerful lobbying groups in American politics, pushing for an expansive view of Second Amendment rights and fighting any measure meant to restrain gun ownership.

Its success meant that firearms increasingly defined the terms of American political debate and the nation’s very culture. People began to regularly carry firearms at demonstrations and protests. In the face of terrifying and deadly school shootings, children learned their ABCs along with how to hide and defend themselves. And the steady drumbeat of bloodshed continued: This year alone has seen at least nine mass shootings, violence in which four or more people died, not including the attacker.

Still, the N.R.A. had suffered setbacks, defections and internal strife in recent years, even as the debates over gun rights increasingly moved to the state level, with more conservative states seeking to enhance access to such weapons and liberal bastions, like New York, trying to stem the tide.