Nearly 25 years after rap star Jam Master Jay of Run-DMC was killed, a man admitted in court on Monday to a role in the shooting that stymied investigators for decades.

Jay Bryant pleaded guilty to a federal murder charge, telling a judge that he helped other people get into a recording studio to ambush the DJ, born Jason Mizell.

“I knew a gun was going to be used to shoot Jason Mizell,” Bryant told a federal magistrate. “I knew that what I was doing was wrong and a crime.”

Bryant’s admission...