Benny Blanco started writing the demo of this song with Ed Sheeran when they were on tour in Philadelphia. A month or so later Blanco was with Halsey at his house. He recalled to Billboard
: "I pulled up the demo. Before I knew it, it was done."
Later, Blanco was listening back to a demo of "Eastside" with Halsey's vocals, thinking he'd always wanted to do a song with Khalid. It occurred to him the song would work as a duet. Khalid agreed to come on board but was in Denmark at the time so Blanco recorded him from there. Guitarist and songwriter Andrew Watt then came over to Blanco's LA studio and recorded the riff.