Jason Mizell (January 21, 1965 – October 30, 2002), known as Jam Master Jay, was the founder and DJ of Run-DMC, a highly influential hip-hop group, based in the Queens borough of New York City. Mizell and the other two members of Run-DMC, the MCs Darryl "DMC" McDaniels, and Joseph "Reverend Run" Simmons, were natives of the middle-class black neighborhood of Hollis, Queens.

The Run-DMC songs "Jam Master Jay", "Peter Piper", and "Daryl and Joe" herald Jay's skills.

Jay had played bass and drums in several garage bands prior to joining Run-D.M.C.; on all of their albums from "Raising Hell" on, he has played keyboards, bass, and live drums in addition to his turntable work.

Run-DMC's signature look of black sweatsuits and leather, with black-striped Adidas sneakers, was modeled after JMJ's personal clothing style.

In 1989, Mizell established the label Jam Master Jay Records, which scored a strong success in 1993 with the band Onyx. He also connected Chuck D with Def Jam co-founder Rick Rubin. (Def Jam's other founder, Russell Simmons, is the brother of DJ Run.)

In the 1990s, Mizell suffered the effects of a bad car accident and a gunshot, mirroring similarly tough times for DJ Run (rape charge) and DMC (alcohol abuse).

Mizell remained in his childhood neighborhood his entire life. He founded the Scratch DJ Academy in Manhattan for children interested in DJing.
Mizell was shot in the head and killed in a Merrick Boulevard recording studio in Queens at 7:30 pm on October 30, 2002. The other person in the room, 25-year-old Urieco Rincon, was shot in the ankle.

A 2003 affidavit used by federal agents to obtain a search warrant of the Murder Inc. record label offices has been revealed by TheSmokingGun.com, and the affidavit shows that Kenneth "Supreme" McGriff, a convicted drug dealer and longtime friend of Murder Inc. heads Irv Gotti and Chris Gotti, had been keeping tabs on 50 Cent and was plotting to kill him. The murder was allegedly planned as retaliation for a song that 50 wrote about McGriff's drug history.

"The investigation has uncovered a conspiracy involving McGriff and others to murder a Rap artist who has released songs containing lyrics regarding McGriff's criminal activities," the affidavit reads. "The Rap artist was shot in 2000, survived and thereafter refused to cooperate with the law enforcement regarding the shooting. Messages transmitted over the Murder Inc. pager indicate that McGriff is involved in an ongoing plot to kill this Rap artist and that he communicates with Murder Inc. employees concerning the target."

The affidavit also says that investigators believed that Murder Inc. employees continued to conspire with McGriff in trying to murder 50 after he survived the 2000 shooting.

The New York Daily News has also reported that authorities have investigated whether McGriff targeted slain Run-DMC member Jam Master Jay for death because the deejay defied an industry "blacklist" of 50 Cent that was imposed due to the aforementioned song. The affidavit says, "Law enforcement agents are investigating the possibility that [Jason] Mizell [a.k.a. Jam Master Jay] was murdered for defying the blacklist of 50 Cent."

Mizell was survived by his wife Terri and three children. He is interred in the Ferncliff Cemetery in Hartsdale, New York.