The relationships between conscientiousness, time-on-task, and academic performance were investigated in a mediation model. When simple summated conscientiousness scores were analyzed, relationships with performance and time-on-task were not significant. When factor scores estimated in the context of a model assuming common method bias were analyzed, conscientiousness was a significant predictor of test performance, and time-on-task fully mediated the conscientiousness–performance relationship. The relationships held for conscientiousness alone and for conscientiousness controlling for cognitive ability.