Sewell, 49, will be the first woman police commissioner of the department.
"I bring a different perspective to make sure the department looks like the city it serves," she said.
Adams had long said the next police commissioner would be a woman, but Sewell was a surprise choice.
"This is the personification of emotional intelligence," Adams said of Sewell. "Commissioner Sewell's appointment is a powerful message to girls and young women across the city - there is no ceiling to your ambition."
She beat out other candidates with more experience leading large departments like Philadelphia and Seattle.
"She exudes what it means to be emotionally intelligent, calm, collected, confident," Adams said.
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Sewell promised to break down barriers between the community and police and said, "policing is about people."
She said that she is laser-focused on guns and getting them out of the hands of criminals.
"We'll arrest violent criminals, take them off the street, and help build the cases to keep them off the street," Sewell said.
She said that "plain-clothes units and anti-crime units work" and she is ready to look at what is being done in the department. She said that the right people with the right temperament need to be chosen to work in those departments.
"It is said that the NYPD is the best of the best and we are about to get even better," Sewell said. "I will have the backs of my officers but they must have the backs of the public."
Sewell has served in the Nassau County Police Department for 22 years.
She was promoted to chief of detectives on September 11, 2020, becoming the first African American woman to serve in that role.
She's leaving that post, where she supervised hundreds of detectives, for a major promotion to lead a force of 36,000 come January 1.
"Scaling up the skills you need to lead, you can scale up," Adams said. "We need to think differently and the skills that Chief Sewell, who will be the chief of the department, the skills that she has, everywhere that she traveled to in the police department in Nassau she improved."
He added, "This is a real winner, we have a real winner in our city."
Sewell clearly did well in the interview process which culminated with a mock press conference. The scenario was a white cop who had shot an unarmed Black man.
Adams said the goal of the mock press conference was to see if the media could shake her and rattle her, and he found her to be very composed. He wants the people of the City of New York to feel secure under her leadership.
PBA President Pat Lynch released a statement saying, "New York City police officers have passed our breaking point. We need to fix that break in order to get our police department and our city back on course. We look forward to working with her to accomplish that goal."
The head of the detectives union added, "It's no surprise to this union that the new police commissioner is Nassau County's current chief of detectives, and we have no doubt Chief Sewell knows how hard our members work."
The official announcement happened Wednesday morning at the Queensbridge Houses where Chief Sewell grew up.
"To all the little girls within the sound of my voice there is nothing you can't do and no one you can't become," Sewell said. "It is said the NYPD is the best of the best. We're about to get even better."
"My shoes are laced up, I'm ready to get to work," she added.
Sewell is a 22-year veteran of the force and was a longtime member of the hostage negotiations team.
She is also a graduate of the FBI National Academy.
The announcement came just days after Adams selected David Banks for the position of chancellor of the New York City Department of Education.
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