The son of Disney Studios Producer Louis Debney (Zorro, The Mickey Mouse Club), John was born and raised in Glendale, California, nearby to Disney. He began guitar lessons at age six and played in rock bands in college. Debney earned his B.A. degree in Music Composition from the California Institute of Arts in 1979. Two weeks after graduating from CalArts, he got a job at copying department at Disney. One day, Buddy Baker saw him and had him arrange music that would later be used for different pavilions and rides at EPCOT Center (at Walt Disney World in Florida). After three years at Disney, he freelanced for television Composer Mike Post. Debney furthered his hands-on training by working with Hanna-Barbera Composer Hoyt Curtin. After this, Debney went on to score television projects as diverse as Disneyland, Star Trek: The Next Generation, Star Trek: Deep Space Nine, SeaQuest DSV, A Pup Named Scooby-Doo, The Cape, The Lazarus Man, Piggsburg Pigs!, The Further Adventures of SuperTed, Doctor Who, Cagney and Lacey, Tiny Toon Adventures, The Young Riders, The New Yogi Bear Show, Police Academy: The Animated Series, Fame, Captain Power and the Soldiers of the Future, Dragon's Lair, Freshman Dorm, Pop Quiz and Dink, the Little Dinosaur, for which he won an Emmy for Best Main Title. In the early 1990s, Debney began to score indie films and Disneyland attractions. In 1991, Debney composed the music for Phantom Manor in Disneyland Paris and SpectroMagic at Magic Kingdom. In 1993, he scored his first studio feature, the Disney comedy Hocus Pocus starring Bette Midler.
Debney has also composed scores for the video games Lair and The Sims Medieval. In 2010, he composed the theme music for the Nickelodeon television series Supah Ninjas.