Keechant Sewell, commissioner of the New York Police Department, said Monday she would resign after less than 18 months, giving no reason for the abrupt end to a tenure during which she won over many in the rank and file even as she jockeyed for position against other appointees and top officers.

Ms. Sewell, who was appointed to her position by Mayor Eric Adams and started in 2022, was the first woman to head the nation’s largest police force. He had promised as a candidate to name a woman to lead the public safety agency where he was an officer for 22 years, giving her the power to rethink policing after bitter protests against police brutality and racism.

The mayor said in a statement on Monday that Ms. Sewell had worked tirelessly and that “New Yorkers owe her a debt of gratitude.” But Ms. Sewell, in an email to the department announcing her resignation, did not mention the mayor at all.

Instead, she focused on the qualities of line officers, calling them hard-working public servants and praising their “compassion, heroics and selflessness.” Ms. Sewell did not say when she would be leaving, and the mayor did not say when a replacement would be chosen.

Ms. Sewell was an enigmatic figure, hardly straying from her script at news conferences and revealing little of her personality, in contrast to her voluble predecessors. When asked in an interview what book she was currently reading, she replied, “stats.” This year, Philip Banks III, the deputy mayor for public safety, began giving widely publicized weekly, live-streamed public briefings — a role that would typically belong to the police commissioner.

Mr. Banks’s appointment had been surrounded by questions over whether his 2014 resignation from the department while the subject of a federal corruption investigation would hamper his credibility and ability to perform the job. He had been named as an unindicted co-conspirator in a corruption investigation that produced several convictions, including that of a Police Department chief who had served as his top aide.

Several senior current and former police officials said that Commissioner Sewell had been undermined since early in her tenure by the mayor and some of his senior aides, including Mr. Banks and Timothy Pearson, a senior adviser to the mayor.

One city official who spoke recently with Ms. Sewell said that Mr. Banks behaved like a shadow police commissioner, and that Ms. Sewell felt frustrated and undermined by having to defend his decisions.

Reached for comment on Monday, Mr. Banks said he was too busy to talk. When asked about the accusations that he had sidelined Ms. Sewell, he abruptly ended the call.

In the interview last month, Ms. Sewell was asked about the role Mr. Banks played in the department, and said she respected his experience. But Ms. Sewell made clear that she was in charge.

“This is my police department,” she said.

She said she was uncomfortable speaking about herself, and was loath to discuss internal politics, preferring to discuss morale among officers and their relationships with the New Yorkers they serve.

On Monday, Ms. Sewell wrote in her email to the department: “I will never step away from my advocacy and support for the N.Y.P.D., and I will always be a champion for the people of New York City.”

Ms. Sewell also had to navigate difficult politics within the agency. Last month, she set a proposed punishment for one of the department’s top chiefs: She moved to strip Jeffrey Maddrey, the highest-ranking uniformed officer, of 10 vacation days, after accusations that he interfered with the arrest of a retired officer who chased three boys while he was armed.

Chief Maddrey planned to fight the charges in a department trial. That set up a battle over what would constitute an unacceptable use of power for the person charged with leading the thousands of rank-and-file officers.

Chief Maddrey is a close associate of the mayor and was handpicked by him, several officials have said, and he does not regularly talk to the commissioner. He frequently calls the mayor directly, bypassing Ms. Sewell, the officials said.

Mr. Adams was asked at an unrelated news conference Monday about whether he required Ms. Sewell to win his approval for relatively low-level personnel decisions. The mayor said he requires that of all department heads. “The people of the City of New York elected one mayor, Eric Adams,” he said. “That’s who they elected. Every agency in the city comes to me with a proposed leadership.”

Despite Ms. Sewell’s rocky relations in the administration, she made a strong impression on the rank and file during her short tenure.

In January 2022, less than a month after she was sworn in, she gave the first of two stirring funeral speeches for Detectives Jason Rivera and Wilber Mora, who were fatally shot when they responded to a domestic disturbance call in Harlem.

Ms. Sewell visited precincts and was so appalled by the conditions of the break rooms, where officers nap during long shifts on worn furniture in rooms with peeling paint, that she ordered they be renovated. During contract negotiations, she agreed to a work schedule for police officers where they would work longer days but shorter weeks, a key concession that the union had been seeking for years.

But the critics said that Ms. Sewell’s department also resorted to unacceptable tactics on the street.

Last week, a court-appointed monitor said in a report that anti-crime units were still stopping, frisking and searching too many people unlawfully — almost all of them people of color — despite assurances from Mayor Adams that new policies and training would end the practice. Almost all of the stops made by the rebranded “neighborhood safety teams” analyzed in the report — 97 percent — were of Black or Hispanic people, and 24 percent of the stops were unconstitutional.

Patrick J. Lynch, president of the Police Benevolent Association, said Ms. Sewell made a significant impact in a short time. The union named her person of the year in 2022, the first time it had ever given that award to a sitting commissioner.

“She cared about the cops on the street and was always open to working with us to improve their lives and working conditions,” Mr. Lynch said in a statement. “There are still enormous challenges facing the N.Y.P.D. Her leadership will be sorely missed.”

Her ease with officers was evident during a recent promotion ceremony at the police academy in Queens. She spent more than an hour after the ceremony posing with officers and their families, who lined up outside a gymnasium to take pictures with her.

She winked at nervous-looking officers, beamed at their parents and spouses, and handed out heavy medallions inscribed with her name and title to their children.

Kamillah M. Hanks, a City Council member from Staten Island who is the chairwoman of the Committee for Public Safety, which oversees the department, said she was “saddened” by the commissioner’s departure.

“We had a productive partnership,” Ms. Hanks said in a statement. “As the first Black woman police commissioner, she is a trendsetter and role model to many.”

Ms. Sewell had previously been Nassau County’s chief of detectives, working in a department that had about 2,400 uniformed officers, less than a tenth of the size of the New York Police Department’s uniformed contingent.

In 23 years with the Nassau Police Department, Ms. Sewell, who grew up in Queens, worked in the narcotics and major cases units, and as a hostage negotiator. She was promoted to chief of detectives in September 2020.

Ms. Sewell, who owns a home in Valley Stream but began renting an apartment in the city after she was appointed commissioner, talked last month about how strange it was to be recognized at supermarkets and on the street.

“I’m surprised at the level of privacy that I’ve lost,” she said. But Ms. Sewell said she understood the attention would be fleeting.

“This is New York. They’ll forget about me when I do leave,” she said. “We all only have these positions for a limited amount of time.”

Jeffery C. Mays and William K. Rashbaum contributed reporting.