Mastery behaviours -- seeking out challenging tasks and continuing to work on them despite difficulties -- are integral to achievement but difficult to measure with precision. The current study reports on the development and validation of the computer-based persistence, effort, resilience, and challenge-seeking (PERC) task in two demographically diverse samples of adolescents (total N = 3,188). We present evidence for convergent validity with self-reported mastery behaviours and learning mindsets, discriminant validity from theoretically unrelated constructs, and incremental predictive validity for grade point average (GPA). Results suggest that the PERC task may be a useful tool for assessing mastery behaviours at scale.