Few thought it would be easy when Laura Kavanagh took the reins of the nation’s largest fire department in November as its first female leader.

Most of the 33 men who had run the New York Fire Department, dating back to 1883, were older, with an up-through-the-ranks pedigree. But Ms. Kavanagh, 40, was the youngest in over a century. She had an Ivy League degree, but was not a native of New York, a background that has long been assumed as a prerequisite. And, though she been a civilian department official for years, she had never been a member of a fire company.

This week, in a kind of culmination of Ms. Kavanagh’s tumultuous early tenure, four of her top chiefs filed a lawsuit against her and the department that took aim at her experience, repeatedly leaning on the same phrase: “Never a day as a firefighter.”

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The suit claims that three of the officials were unfairly demoted, putting New Yorkers at risk. Ms. Kavanagh on Tuesday evening dismissed that idea.

“We are putting together a new team, and that’s always difficult, but it’s also what every commissioner before me has done,” she told Fox 5 News.

Ms. Kavanagh was named to her job by Mayor Eric Adams with great fanfare last year in a speech that made clear he considered her appointment a historic moment.

“When you say, as men in this agency, that you will follow the leadership no matter what the gender may be, you are raising your standard as firefighters and you are willing to face the future head on,” he said. “You earned the right to be called the bravest not only because you’re willing to go into the flames of uncertainties in buildings, but the flames that keep our futures burned down over and over again.”

Despite that optimistic start, tension built swiftly in the department. A month ago, Ms. Kavanagh called her top chiefs into a boardroom in the Fire Department’s headquarters in Downtown Brooklyn.

She and her chiefs aired differences. But three who had been butting heads with Ms. Kavanagh were noticeably absent.

They would soon be demoted, a move that pitted Ms. Kavanagh against a group of experienced chiefs with hundreds of years of firefighting experience among them.

The four plaintiffs in the lawsuit claim that Ms. Kavanagh unfairly stripped them of power in retaliation for bringing up safety concerns. The suit said more chiefs stepped down, citing claims that they were no longer able to do their jobs properly.

The suit depicts Ms. Kavanagh’s brief tenure as defined by discord with her top chiefs, and includes charges that her leadership has left the department with “an unimaginable level of unpreparedness.”

It portrays Ms. Kavanagh as being outside the culture of the department, and her opponents as an old guard of veteran chiefs, several of whom responded to the Sept. 11, 2001, attacks and other major calamities.

The lawsuit, filed in State Supreme Court in Brooklyn, claims that the shake-up has created a “grave risk,” because it leaves the city with no staff chiefs to properly oversee five-alarm fires, and only a handful of officials with four-alarm command experience.

Jim Walden, the four chiefs’ attorney, said chiefs were sidelined by being assigned to menial and humiliating posts far below their capabilities.

On Tuesday evening, Ms. Kavanagh said that despite the demotions, leadership continues uninterrupted and her staff remains intact.

“The idea that any of these positions are open is completely ridiculous,” she said. “The chiefs that I work alongside every day are completely committed to the mission of keeping our members safe and the city safe. They are all in their positions and doing the extraordinary work that they do every day.”

Ms. Kavanagh dismissed claims that the shake-up has compromised the city’s safety.

“Every person we have on staff is extraordinarily experienced, they have decades of service and they are still serving in their positions,” she said, adding that changes in leadership positions are hardly uncommon for new commissioners.

A spokesman from the city’s Law Department said Ms. Kavanagh has done nothing wrong. “The personnel decisions made by the F.D.N.Y. commissioner were lawful and within her authority,” said the spokesman, Nicholas Paolucci.

Ms. Kavanagh, who has worked on the campaigns of former Mayor Bill de Blasio and others, joined the department as a civilian. She became commissioner in November after serving eight months last year as acting commissioner and an additional seven years in other civilian positions in the department.

The commissioner took office promising a more diverse work force, a task that required confronting a department that has struggled for decades to improve a culture where racism, sexism and harassment have been present.

Ms. Kavanagh received support from associations supporting Black and female firefighters. She also supported legislation recently passed by the City Council seeking to remedy race and gender diversity deficits so chronic that in 2011 they led to the appointment of a federal monitor.

This week’s suit disparaged her as a “political operative.”

Glenn Corbett, an associate professor of fire science at the John Jay College of Criminal Justice in Manhattan, said turmoil is almost to be expected in a department where the top chiefs, who value hands-on firefighting experience, are now taking orders from a politically appointed commissioner “who never stretched a hose line into a building or threw up a ladder.”

“When you bring in somebody who’s never been involved in that, there is going to be a level of skepticism,” he said. “‘Is this person going to be advocating on our behalf on the things we need?’”

“Her biggest challenge is gaining the confidence of the rank and file and senior leadership.”