Jon-Adrian Velazquez is the subject of a new book, a podcast and a coming documentary series.

He has spoken at dozens of justice events and was invited to the White House in 2022 to meet President Biden. He has famous friends like the actor Martin Sheen and played himself alongside the actor Colman Domingo in “Sing Sing,” a film released this summer about a prison theater program.

All while the legal system considered him a convicted killer.

That changed Monday morning, when a judge in Manhattan overturned Mr. Velazquez’s second-degree murder conviction. The charge had caused Mr. Velazquez, 48, to serve nearly 24 years in prison for the 1998 shooting of Albert Ward, a retired police officer, during an attempted robbery of an illegal gambling parlor that Mr. Ward ran in Harlem.

The judge, Abraham Clott, said that in light of new DNA evidence, he would set aside Mr. Velazquez’s verdict, a decision supported by Manhattan prosecutors.

Mr. Velazquez’s face remained stern, despite a jubilant crowd of supporters both inside the courtroom and outside the building.

“This isn’t a celebration,” he said outside, adding, “This is an indictment of the system.” He wore a baseball cap bearing the words “End of an error.”

Ruthless police and prosecutorial tactics during the high-crime, crack-ravaged 1990s in New York City resulted in scores of wrongful convictions. Even decades later, a steady stream of old, flawed cases is still being overturned with the help of advances in DNA testing, innocence groups and conviction review units.