Kennedy was born in Upper Darby, Pennsylvania. He attended and graduated from Monsignor Bonner High School in 1988. He is an Irish American comedian and actor. After studying acting in college, he began his career as a Hollywood, California extra and honed his skills at voice impersonations.

This skill led to, among other things, an unorthodox success in the way of creating a false persona, screen agent "Marty Power" (a fictitious person). Posing as Marty Power over the telephone, Kennedy attracted the attention of real agents and managers who would later book his performances. He eventually landed his first major motion picture role as Randy Meeks in the films Scream, Scream 2, and made a cameo in Scream 3. As a performer of stand-up comedy, Kennedy is best known for his sketch performances on his television reality show, The Jamie Kennedy Experiment (which became the WB Network's highest ranking new show in 2002, but which was cancelled in April 2004 due to falling ratings).

He formed a production company called Wannabe Producers with Josh Etting through which he has produced shows The Jamie Kennedy Experiment, Blowin' Up as well as The WB shows The Starlet and Living with Fran.

Following great success of Malibu's Most Wanted (2003), which he both wrote and starred in, Kennedy co-wrote the MTV show Blowin' Up (2006) featuring his friend Stu Stone.

During the time when he was working on Malibu's Most Wanted, Kennedy wrote a pseudo-comedic autobiography entitled, "Wannabe". It chronicles his life during his life in the hollywood slums, his lame attempts at stardom, and then his gradual rise to fame. The book was never well known, but the critics that did review it claimed it was well written, and entertaining.

He has also performed minor roles in a number of other films.