The longtime head of the National Rifle Association said Friday he is resigning, just days before the start of a civil trial regarding allegations he diverted millions of dollars from the powerful gun rights organization to pay for personal travel and other lavish perks.

The executive vice president and chief executive officer, Wayne LaPierre, said his departure is effective January 31. The trial in Attorney General Letitia James’s lawsuit against him, the NRA, and others who have served as organization executives is scheduled to start on Monday. Mr. LaPierre and a previous NRA president, Oliver North, are among the witnesses expected to testify.

Mr. LaPierre, 74, has led the NRA’s day-to-day operations since 1991, acting as the face and vehement voice of its gun rights agenda. He once warned of “jack-booted government thugs” seizing guns, called for armed guards in every school after a spate of shootings, and condemned foes backing gun control measures as “opportunists” who “exploit tragedy for gain.”

In recent years though, the NRA has been beset by dwindling membership and financial troubles, along with lingering questions about Mr. LaPierre’s leadership and spending. One of Mr. LaPierre’s top lieutenants, Andrew Arulanandam, will assume his roles on an interim basis, the organization said.

A former NRA board member who is set to testify at the upcoming trial in New York, Phillip Journey, said Mr. LaPierre’s resignation doesn’t resolve the open questions before the court nor remedy what he sees as persistent rot within the gun lobbying group. “Honestly, the grifters are a snake with many heads and this is just one,” said Mr. Journey, who is a Kansas judge running to retake a seat on the NRA’s board.

Mr. Journey also testified at the NRA’s bankruptcy trial in Texas and said he anticipates there is enough evidence for the New York attorney general to prove her case. “It’s a tragic end to a career that had many high points,” Mr. Journey said of Mr. LaPierre stepping down. “It’s one of his own making.”

“With pride in all that we have accomplished, I am announcing my resignation from the NRA,” Mr. LaPierre said in a statement released by the organization. “I’ve been a card-carrying member of this organization for most of my adult life, and I will never stop supporting the NRA and its fight to defend Second Amendment freedom. My passion for our cause burns as deeply as ever.”

Ms. James, who is a Democrat, accuses Mr. LaPierre and other executives of illegally diverting tens of millions of dollars from the NRA and spending organization funds on personal trips, no-show contracts, and other questionable expenditures. Mr. LaPierre is accused in the lawsuit of spending millions on private jet flights and personal security and accepting expensive gifts — such as African safaris and use of a 107-foot yacht — from vendors.

He is also accused of setting himself up with a $17 million contract with the NRA should he exit the organization, spending NRA money on travel consultants, luxury car services, and private jet flights for himself and his family — including more than $500,000 on eight trips to the Bahamas over a three-year span.

Ms. James is seeking to ban Mr. LaPierre and the other executives from serving in leadership positions of any not-for-profit or charitable organization conducting business in New York, which would effectively remove them from any involvement with the NRA.

Some of the NRA’s excess spending was kept secret, the lawsuit said, under an arrangement with the organization’s former advertising agency, Ackerman McQueen. The advertising firm would pick up the tab for expenses for Mr. LaPierre and other NRA executives and then send a lump sum bill to the organization for “out-of-pocket expenses,” the lawsuit said.

Mr. LaPierre has defended himself, saying in previous testimony that cruising the Bahamas on a yacht was a “security retreat” because he was facing threats after mass shootings. He conceded not reporting the trips on conflict-of-interest forms, testifying: “It’s one of the mistakes I’ve made.”

“He’s been the leading political force of the NRA for over 30 years,” a political science professor at the State University of New York-Cortland and author several books on gun politics, Robert Spitzer, said. “He has been the leading edge of the very sharp political voice of the organization.”