The 18-year-old scored 88.40 to make history in what was the fifth and the final stop of the World Cup season.
She came ahead of Sweden's Emma Dahlstrom and Swiss Mathilde Gremaud.
Boston-born Atkin, who initially competed for the US before switching to Great Britain aged 15, was making her 15th appearance at a World Cup event.
Atkin will be competing at the Freestyle World Championships in Sierra Nevada, Spain (9-19 March). The event will be live on the BBC Sport website, app, connected TV and red button.
Izzy Atkin became the first British woman to win a World Cup ski slopestyle event as she took gold in Silvaplana, Switzerland.