A man who was charged with murder in the fatal stabbing of an activist on a Brooklyn street in 2023 has pleaded guilty to the killing, which prosecutors called “random and unprovoked.”

The man, Brian Dowling, 20, appeared on Wednesday before Justice Danny K. Chun of State Supreme Court in Brooklyn and accepted an offer of 20 years to life in prison in exchange for the guilty plea in the death of the activist, Ryan Carson, 32. He will be sentenced on Feb. 19, according to a news release from the Brooklyn district attorney’s office.

Mr. Dowling’s lawyer did not immediately respond to a request for comment.

Claudia Morales, Mr. Carson’s girlfriend, said that the plea represented the “end of a very long journey.”

“There was relief in knowing that part was over, but it is also never going to be over,” she said.

Mr. Carson was a well-known community organizer who worked as a campaign manager for the New York Public Interest Research Group and ran an advocacy campaign to end drug overdose deaths. He also wrote poetry.

His murder was met with an outpouring of grief from advocates and elected officials.

“This tireless defender of his neighbors was stolen from us,” Chi Ossé, a city councilman who represents Bedford-Stuyvesant and Crown Heights, said on X. His funeral in Massachusetts, where he grew up, “felt like a funeral for a senator,” Assemblywoman Emily Gallagher, who represents a North Brooklyn district and was a friend of Mr. Carson’s, told The New York Times.

Mr. Carson and his girlfriend were sitting on a bus stop bench on Malcolm X Boulevard in Bedford-Stuyvesant just before 4 a.m. on Oct. 2, 2023, when a man, later identified as Mr. Dowling, walked past them and began kicking parked scooters, the police said. The couple stood up and began walking in his direction when the man suddenly turned and started yelling at them.

According to prosecutors, there was “clear” video footage of the fatal encounter, which showed Mr. Dowling shouting “I’m going to kill you” just before swinging a knife at Mr. Carson, who was trying to de-escalate the situation.

When Mr. Carson tripped and fell while backing away, Mr. Dowling stabbed him three times, including once in the heart, the police said.

The police said Mr. Dowling had kicked Mr. Carson and threatened his girlfriend as he lay on the sidewalk, bleeding out. They also said that a woman who seemed to be Mr. Dowling’s girlfriend was seen with him on the street before the stabbing and appeared at the scene soon after, calling Mr. Dowling by name and apologizing to the couple.

Mr. Dowling threw the knife, but returned to retrieve it moments later, prosecutors said, adding that investigators had later found it hidden under some greenery nearby.

Mr. Dowling surrendered later that week. A search of his apartment turned up numerous knives with handles similar to the murder weapon and clothes matching what the killer was wearing in the video, according to prosecutors.

“Many of us still shudder when recalling the horrific video showing this defendant viciously attacking Ryan Carson and stabbing him to death for no reason at all,” Eric Gonzalez, the Brooklyn district attorney, said. “Ryan was passionate about making our city a better place, and I hope that today’s outcome will bring his loved ones a small sense of closure.”