Jason William Mizell (January 21, 1965 – October 30, 2002), better known by his stage name Jam Master Jay, was an American musician, record producer and DJ. He was the DJ of the influential hip hop group Run-DMC. During the 1980s, Run-DMC became one of the biggest hip hop groups, credited with breaking hip hop into mainstream music. Mizell was murdered in his Queens recording studio in 2002 and his case was left unsolved until February 2024, when two men named Ronald Washington and his godson Karl Jordan Jr., were convicted for the 2002 drug-related murder, with a third man, Jay Bryant, facing a separate trial in 2026.

Jason Mizell was born in Brooklyn, the son of Jesse Mizell and Connie Thompson Mizell (later Connie Mizell-Perry).

At age three, Jason began playing trumpet. He learned to play bass, guitar, and drums. He performed at his church and in various bands prior to discovering turntablism. After he and his family moved to Hollis, Queens, New York City, in 1975 at the age of 10, he discovered the turntables and started DJing at the age of 13. He was high school friends with Wendell "DJ Hurricane" Fite, known for his 13-year collaboration with Beastie Boys.

As a teenager, Mizell was involved with a group that committed residential burglaries. An encounter with an armed security guard frightened him into stopping the burglaries, and as an adult he was known for discouraging criminal activities among his friends and family.

For a time, he lived in Atlantic City, New Jersey, where turntablism pioneer DJ Def Lou Hauck taught him to crossfade. He caught on quickly because of his musical experience and after a year of DJing he felt that he was good enough to play in front of people. Originally calling himself Jazzy Jase, he attended high school at Andrew Jackson High School in New York City's Queens.

He first started playing at parks and later played at bars. He also began throwing small parties around the area. Once he got a pair of Technics 1200s, he improved rapidly, since he was able to practice at night with headphones on when he was supposed to be sleeping.

Mizell became a DJ because he "just wanted to be a part of the band". Prior to joining Run-D.M.C., he played bass and drums in several garage bands. In 1982, he joined Joseph "Run" Simmons and Darryl "D.M.C." McDaniels just after they graduated from high school and agreed to DJ for them. On Run-D.M.C's album Raising Hell, Mizell played keyboards, bass, and live drums in addition to his turntable work. Mizell remained in his childhood neighborhood in Hollis, Queens his entire life.

Despite not being considered to the general public as prominent to Run-DMC as Simmons and McDaniels were, with his public role being Run DMC's DJ and less notable musician, Mizell's role with the rap trio would in fact become more prominent behind the scenes. Mizell was in fact very vital to Run-DMC, as he would give the group their street credential image and would even be considered the Run-DMC member who was most responsible for expanding the worldwide audience of the hip hop genre, as he developed the sound of most Run-DMC songs. In 2012, The Guardian described Mizell as having "created almost every sound that Run and DMC would rap over."