Feldmann started playing music when he was 16 after hearing Mommy's Little Monster by Social Distortion. His first band was Saratoga-based Family Crisis. He moved to Los Angeles in 1988 and formed a band called Electric Love Hogs where he met future Goldfinger bassist Kelly LeMieux. The Electric Love Hogs (which also included Dave Kushner of Velvet Revolver and Bobby Hewitt of Orgy) were signed to Polygram in 1990 and put out one album.
Feldmann formed Goldfinger in 1993 when he met original Goldfinger bassist Simon Williams in a shoe store where they were both working. The group was signed to Mojo/Universal in 1995.
During his time as an A&R Representative for Warner Bros. Records & Maverick Records, Feldmann signed The Used, Destroy Rebuild Until God Shows, Showoff, City Sleeps, Story of the Year, Foxy Shazam, and Mest.
He has done production work and songwriting for The Cab, The Veronicas, Good Charlotte, Mest, The Used, Story of the Year, Ashlee Simpson, ONE OK ROCK, Hilary Duff, Goldfinger, Showoff, The Matches, Atreyu, City Sleeps, Anthony Green, Cute Is What We Aim For, Escape the Fate, Foxy Shazam, All Time Low, Allstar Weekend, Panic! at the Disco, Boys Like Girls, Plain White T's, 5 Seconds of Summer, Blink-182, Black Veil Brides, Andy Black, Sleeping With Sirens, and Stick to Your Guns.
Feldmann is a former A&R Consultant for Red Bull Records. He has signed Itch and Beartooth. In 2016, John became the Vice President of A&R at BMG and also inked a publishing deal with the company to sign and develop new talent. While at BMG, John signed The Faim.Today John runs A&R for Big Noise Collective
In 2016, he produced the seventh studio album for Blink-182 titled California which landed both he and the band their first Grammy nomination for "Best Rock Album". Also in July 2017 John was awarded the "Influencer" Award at the Alt Press Music Awards (APMAs).
He has been nominated for a Grammy for Best Rock Performance for his work with Fever 333 on Made An America.
John started a record label with former president of Vagrant Records and BMG Rights Jon Cohen, and philanthropist Nick Gross called Big Noise and have signed Ashley Tisdale, Half The Animal, Goldfinger, RØYLS, and New Politics.
John started Back to the Beach Festival at Huntington State Beach with Travis Barker and Synergy Global Entertainment founder John Reese in 2018. With headliners Blink-182 the festival sold out in 2019.
Feldmann is a vegan and vocal activist for animal rights. He became a vegetarian after seeing the movie Babe in 1995. He usually has animal rights material to give out at Goldfinger shows. He has spoken on behalf of the animal rights organizations PETA and Action for Animals. Feldmann has appeared in the documentary films Behind The Mask and Skin Trade. He has also appeared in the film Bold Native.