Anxiety disorder is the most prevalent and frequently diagnosed disorder in youth, and associated with serious negative health outcomes. Our most effective prevention programs, however, have several limitations. These limitations can be addressed using game-based interventions1. Results from two randomized controlled trials on the video game MindLight show improvements in anxiety that are maintained up to six months2,3. The game was designed based on evidence-based therapeutic techniques; however, it is unclear if children’s engagement with these techniques actually predict improvements in anxiety symptoms. An important advantage of game-based interventions is that they provide excellent opportunities to isolate therapeutic action mechanisms and test their impact on intervention outcomes. In the current study, on-screen videotaped output while playing MindLight was coded and analysed for forty-three 8 to 12-year old children with elevated levels of anxiety. Results showed that changes in in-game play behaviours representing therapeutic exposure techniques predicted improvements in anxiety symptoms three months later (when children had not played the game for three months). The current study is a first step towards identifying and validating game mechanics that can be used in new applied games to target anxiety symptoms or other psychopathologies with the same underlying deficits.