Born August 24, 1997, in Northampton, England, to a Norwegian mother and British father, Walker moved with his family to Bergen, Norway, at the age of two. A true post-millennial child, Walker grew up surrounded by technology and became fascinated by computers. In 2012, inspired by EDM producers K-391 and Ahrix, whom he'd discovered on YouTube, and film composers Steve Jablonsky and Hans Zimmer, he began to create his own music on his laptop. Working in the genre of "drumstep," a kind of slowed-down drum'n'bass, he eventually uploaded the instrumental track "Fade," which was soon picked up and re-released via the No Copyright Sounds (NCS) YouTube channel and free netlabel that promoted the two aforementioned EDM artists. "Fade" was a huge success, eventually garnering a staggering 65 million hits, and was followed by two more equally cinematic (if somewhat less popular) songs, "Spectre" and "Force."

The majors soon sat up and took notice, and Walker signed to Sony Music in 2015. His first single for the label was "Faded," a reworked version of "Fade" featuring uncredited lead vocals by fey Oslo pop singer Iselin Solheim, who had burst onto the scene only a couple of years before with her song "The Wizard of Us." Driven by Solheim's angelic, ethereal vocal, a melancholy yet catchy melody, and punchy, high-quality production -- as well as a dark, dystopian music video shot in a number of crumbling post-industrial locations in Estonia -- the song topped the charts in Sweden (where it went nine-times platinum), Germany, France, Italy, his native Norway, and elsewhere around the world. He later released a stripped-back acoustic "restrung" version with all the EDM elements taken out. Walker made his live debut at the X Games in Oslo in February 2016, followed by a series of high-profile festival appearances.

In 2017, Walker released the single "Tired," which featured Irish singer/songwriter Gavin James, and followed in 2018 with "Ignite," a collaboration with Julie Bergan, K-391, and Seungri. "Darkside" (featuring Au/Ra and Tomine Harket) and "Diamond Heart" (featuring Sophia Somajo) each hit number one on the Norwegian chart and were later included on his chart-topping debut album, Different World, that December. 2019 brought the singles "Are You Lonely" with Steve Aoki and ISÁK and "On My Way" featuring Sabrina Carpenter and Farruko. Over the next two years, Walker continued to land major streaming hits like 2020's "Alone, Pt. II" and 2021's "Fake a Smile."