Eric Adams, the city’s mayor-elect, announced that he had chosen her for the job on Wednesday, with the two appearing together at a news conference in Queens.

Sewell and Adams will both begin their new positions on January 1. Sewell, 49, will replace Dermot Shea, who is retiring.

Sewell said she will be “laser-focused on violent crime” as commissioner, especially centering on gun violence, which has been trending upward not just in the city but across the nation.

“We are in a pivotal moment in New York as our city faces the twin challenge of public safety and police accountability,” Sewell said at the news conference. “They are not mutually exclusive.”

Being only the third Black commissioner in New York City’s history, Sewell also said she plans to focus on making the NYPD more diverse.

“I bring a different perspective, committed to make sure the department looks like the city it serves, and making the decision, just as Mayor-elect Adams did, to elevate women and people of color to leadership positions,” she said.

“She’s the woman for the job,” Adams, himself a former New York police captain, declared as he appeared with Sewell at the news conference.

“She carried with her throughout her career a sledgehammer, and she crushed every glass ceiling that was put in her way,” Adams said. “Today, she has crashed and destroyed the final one we need in New York City.”

Adams had promised on the campaign trail that he would hire a woman as commissioner. Other potential candidates included former Seattle Chief Carmen Best, Philadelphia Commissioner Danielle Outlaw, former Newark Chief Ivonne Roman and NYPD Chief of Patrol Juanita Holmes.

Adams praised Sewell for her “emotional intelligence,” describing her as “calm, collected, confident” and someone who has risen through the ranks.

It has been decades since a Black person ran the NYPD, with Benjamin Ward and Lee Brown, who served in the 1980s and 1990s, preceding Sewell. She will inherit a police department in flux. The NYPD has struggled to keep crime down a few years after achieving record lows.

The rise, particularly in shootings and killings, is part of a national trend in the wake of the pandemic, but police officials have also blamed state reforms that eliminated pretrial detention for many charges. There is little evidence that the reforms have resulted in more crime.

Adams, the co-founder of 100 Blacks in Law Enforcement Who Care, an advocacy group that sought criminal justice reform and spoke out against police brutality, has pledged new strategies to fight crime, including the return of foot patrols.

He has pushed back against progressive calls to defund the police and has defended the controversial stop-and-frisk police strategy as a useful tool that has been abused. He has also pledged to diversify the NYPD’s ranks.

Among about 35,000 uniformed members of the department, about 45% are white, 30% are Hispanic, 15% are Black and 10% are Asian.

Sewell on Wednesday reiterated that promise to diversify the force.

Sewell was named Nassau County’s chief of detectives in September 2020, overseeing a staff of about 350 people. The NYPD has about 35,000 officers.

Adams acknowledged Sewell has been leading a much smaller force in her current role, but said Wednesday she helped make Nassau County one of the safest communities in the country.

Sewell has overseen Nassau County’s detectives, including its homicide squad and special victims squad, for about a year. Before that, she oversaw the department’s professional standards bureau and internal affairs, according to a report last year in Newsday.

She started with the department as a patrol officer in 1997 and worked her way up the ranks to become a precinct commander, to head the department’s bureau of major cases and to serve as the chief hostage negotiator.

The New York Post first reported the selection of Sewell on Tuesday night.

The Associated Press contributed to this report.