Mr. Garner’s last words, “I can’t breathe,” galvanized the Black Lives Matter movement.

The New York City police officer whose chokehold led to Eric Garner’s death in 2014 was fired from the Police Department and stripped of his pension benefits on Monday, ending a bitter battle that had cast a shadow over the nation’s largest police force.

Commissioner James P. O’Neill’s decision to dismiss the officer, Daniel Pantaleo, came five years after Mr. Garner’s dying words — “I can’t breathe” — helped to galvanize the Black Lives Matter protests that led to changes in policing practices in New York and around the country.

Officer Pantaleo had held on to his job as the Staten Island district attorney and the Justice Department declined to charge him with a crime in the face of calls by the Garner family and their supporters that the city punish him and other officers involved.

Commissioner O’Neill dismissed Officer Pantaleo just over two weeks after a police administrative judge had found him guilty of violating a department ban on chokeholds.

The commissioner gave an emotional explanation laced with sympathy not just for Mr. Garner, but for Officer Pantaleo, and said he had agonized over the decision. He said he might have made similar mistakes if he had been in Mr. Pantaleo’s place, and noted that Mr. Garner should not have resisted arrest when he was stopped and accused of selling untaxed cigarettes.

Still, the commissioner said, Officer Pantaleo had failed to relax a grip on Mr. Garner’s neck after he tackled him to the ground, and his recklessness triggered a fatal asthma attack.“The unintended consequence of Mr. Garner’s death must have a consequence of its own,” Commissioner O’Neill said. “It is clear that Daniel Pantaleo can no longer effectively serve as a New York City police officer.”Commissioner O’Neill’s voice wavered as he delivered a decision that he said had been difficult to make and was long overdue. He said he expected it would not only fail to deliver justice to the Garner family, but would also make his own rank-and-file question whether he supported them.

“Every member of law enforcement in this country that works to keeps this country safe and this city safe, looked at that and said, ‘That could possibly be me,’” said Commissioner O’Neill, who was a police officer for nearly 34 years before he was appointed commissioner. “It’s in my DNA. It’s who I am. But as police commissioner, I have to think about the city. I have to think about the rules and regs of the N.Y.P.D. and make sure they’re followed.”

[Read more: How the police commissioner grappled with the Pantaleo decision.]

The leader of the city’s largest police union denounced the decision, saying Commissioner O’Neill had bowed to “anti-police extremists” and that Officer Pantaleo’s dismissal sent a message that the city did not stand behind its officers when they make arrests.

“We are urging all New York City police officers to proceed with the utmost caution in this new reality, in which they may be deemed ‘reckless’ just for doing their job,” the Police Benevolent Association president, Patrick J. Lynch, said in a statement. “We will uphold our oath, but we cannot and will not do so by needlessly jeopardizing our careers or personal safety.”

Mr. Garner died on July 17, 2014, after Officer Pantaleo tackled him from behind, then, along with other officers, pressed him down on the pavement. Police supervisors had sent Officer Pantaleo and his partner to arrest Mr. Garner because they suspected he was selling loose cigarettes near Tompkinsville Park in Staten Island.

The case had defined the Police Department’s relationship with the public under Mayor Bill de Blasio, who campaigned for office on a promise to reverse the aggressive policing of low-level crime and disorder — known as the “broken windows” strategy — that his predecessor had championed. The mayor had come under intense criticism for refusing to fire Officer Pantaleo.

At a news conference in City Hall, Mr. de Blasio presented the decision as the final resolution of what had been one of the most fraught political and cultural sagas of his mayoralty. The firing of Officer Pantaleo “ended a chapter that has brought our people so much pain and so much fear these last five years,” he said.

“Today we have finally seen justice done,” said Mr. de Blasio, who is running for president on his credentials as a progressive Democrat. “We must devote ourselves to this simple goal: No one should have to go through the agony that the city has gone through here. Let this be the last tragedy.”

Mr. de Blasio blamed the Justice Department for taking five years before deciding not to pursue civil rights charges, saying “the place that was synonymous with making things right failed us.” The mayor also sought to calm police officers angered by Officer Pantaleo’s firing, urging New Yorkers to thank them for their efforts.

At a new conference a few blocks away, Mr. Lynch said rank-and-file officers were outraged and “brokenhearted.” He called on Governor Andrew M. Cuomo to remove Mr. de Blasio and Commissioner O’Neill from office for reckless leadership. “There’s no confidence for the leadership at City Hall and One Police Plaza,” he said. “The leadership has abandoned ship and left our police officers on the streets, alone.”

Officer Pantaleo’s lawyer, Stuart London, said his client plans to sue in state Supreme Court to be reinstated. “He is disappointed, upset, but has a lot of strength,” Mr. London said. “He wants to go forward.”

Outside Police Headquarters, Mr. Garner’s mother, Gwen Carr, joined supporters in the afternoon — some holding signs that read “FIRED” over Officer Pantaleo’s photograph — to demand that the mayor and the police commissioner also fire the 11 other police officers who took part in her son’s arrest. Only one — Sgt. Kizzy Adonis, who was the first supervisor to arrive on the scene — faces disciplinary charges.

“They all need to lose their jobs,” she said.

“Pantaleo, you may have lost your job, but I lost a son,” she added. “You cannot replace that. You can get another job, maybe at Burger King.”Earlier, other members of Mr. Garner’s family said they would continue to press for congressional hearings into his death and for state legislation making it a crime for a police officer to use a chokehold.

Emerald Snipes Garner, Mr. Garner’s daughter, thanked Commissioner O’Neill “for doing the right thing” at a news conference in Harlem. “You finally made a decision that should have been made five years ago,” she said.

For years, the Garner family, some elected officials and critics of the Police Department have said Mr. Garner’s death stemmed from the department’s disproportionate response to crime and disorder in black and Latino neighborhoods.

And as national protests grew, Officer Pantaleo became a symbol of longstanding problems with how the police treated people — mostly black and Latino — suspected of low-level crimes.

After Mr. Garner’s death, the Police Department scaled back the heavy enforcement of low-level crimes. But Officer Pantaleo’s continued employment on the police force still infuriated Mr. Garner’s family and their supporters. They lobbied for the officer to be fired and stripped of his pension, and put pressure on Mr. de Blasio to make it happen.Speaking to reporters at Police Headquarters, Commissioner O’Neill said he had tried to be fair and impartial and to make the decision without regard to political considerations. He noted that Officer Pantaleo had been sent to arrest Mr. Garner as part of an effort to stop drug dealing and other crime around Tompkinsville Park.

A Staten Island grand jury and federal civil rights prosecutors declined to bring criminal charges against Officer Pantaleo, and it was not until May, when the Police Department held a disciplinary trial, that the public heard testimony about what had happened.

On Aug. 2, a department judge, Deputy Commissioner Rosemarie Maldonado, found him guilty of reckless assault following the trial at Police Headquarters.

The judge’s ruling was a major victory for the Civilian Complaint Review Board, a city agency that investigates abuse and misconduct allegations against the police and prosecutes some disciplinary cases under an agreement with the department. Its prosecutors had the unusually high burden of proving that Officer Pantaleo’s conduct constituted a crime in order to avoid an 18-month limit on bringing misconduct charges.

Fred Davie, the agency chairman, said that Officer Pantaleo’s termination “does not make the death of Eric Garner any less harrowing. But it is heartening to know that some element of justice has been served.”

Judge Maldonado affirmed in her 46-page decision what many people, including federal prosecutors, believe the video plainly showed: Officer Pantaleo’s initial grip on Mr. Garner slipped as the two men grappled and became a chokehold, which the department banned more than two decades ago. But, like the local grand jury and federal prosecutors before her, she was not persuaded that the chokehold was intentional.

Still, she wrote Officer Pantaleo’s use of a chokehold “fell so far short of objective reasonableness that this tribunal found it to be reckless.”