Grundström returned to the Marlies for the 2018–19 season. He recorded 29 points in 42 games for the club. On 28 January 2019, Grundström (alongside Sean Durzi and a 2019 first-round pick) was traded to the Los Angeles Kings in exchange for Jake Muzzin. He was initially assigned to their American League affiliate, the Ontario Reign, but was recalled by the Kings on 9 March. He made his debut that night, and scored his first career NHL goal in a 4–2 loss to the Arizona Coyotes.