An East Harlem mom burst into tears in a Manhattan courtroom yesterday as her 17-year-old daughter’s stray-bullet killer was convicted of murder.

“It was tears of joy,” Lisa Baez explained after jurors convicted Boris Brown, 22, of murdering Cheyenne Baez with depraved indifference when he fired randomly into the crowded courtyard of a housing project on East 128th Street in October 2010.

“Justice is served, so we’re at peace now knowing that he is reaping what he sowed,” the mom said.

The Supreme Court jury acquitted Brown’s co-defendant, Devon Coughman, 23, of murder but found him guilty of weapons possession for being with Brown despite knowing his pal possessed a gun.

“He had no knowledge that Brown was going to shoot wildly into a crowd and kill that young girl,” said Coughman’s lawyer, Todd Spodek.

District Attorney Cyrus Vance Jr. said: “The defendants displayed a shocking disregard for human life when they went to the AK Houses with a gun and one of the defendants, Boris Brown, fired into a crowd, murdering an innocent young woman,”

Sentencing is set for June 25. Brown faces up to life for murder, and Coughman can get anywhere from 3 1/2 to 15 years for gun possession.