In 2012, Walker was listening to a song by David Whistle (formerly known as DJ Ness), and reached out to the Italian DJ to find out how he produced his music. He was inspired by EDM producers K-391 and Ahrix, and is also influenced by film composers like Hans Zimmer and Steve Jablonsky. He started all his music on his laptop with FL Studio. In July 2012, with the help and feedback from his fans online, he began pursuing his music production career and slowly began posting his music to YouTube and SoundCloud.
Walker released the track "Fade" on 17 August 2014. The track gained attention after its re-release via the record label NoCopyrightSounds on 19 November. Walker stated that the creation of the track was inspired by K-391 and Ahrix, whose tracks were also picked up by the record label. The track has over 290 million views on YouTube, 700 million plays on Spotify, and 20 million streams on SoundCloud. It has become the most popular track on the channel, and is widely used by YouTubers in their videos. The tracks "Spectre" and "Force" followed in 2015.
Walker signed with MER Musikk under Sony Music Sweden and his next single, "Faded", was a remastered vocal version of "Fade". It was released on 3 December and featured uncredited Naustdal pop singer Iselin Solheim. The single topped the year-end charts in Austria, Germany, Switzerland and Sweden, iTunes charts in 32 countries, as well as entering top 10 in Spotify Global Chart. The music video on YouTube has over 1.5 billion views and 9 million likes, placing it among the top 10 most liked YouTube videos. It has over 748 million plays on Spotify, and is also one of the Top 10 Most Shazamed tracks of 2016. The single also received official remixes from Tiësto, Dash Berlin and Hardwell. He later released an acoustic "restrung" version of the song, with all the EDM elements taken out.