In order to optimize recruitment and the overall outcome of educational programmes, it is crucial to understand personal determinants of achievement. While several cognitive abilities and skills individually predict performance in academic and professional settings, it is less clear how personality translates into performance. This study addresses the impact of the Big Five personality trait, conscientiousness, on academic performance and instructor performance ratings and examines the mediating role of self-efficacy. Analysis of longitudinal data (Time 1: n = 166 (conscientiousness); Time 2: n = 161 (self-efficacy); Time 3: n = 136 (military performance) and n = 156 (academic performance)) from three military academies in Norway showed that conscientiousness was related to both military and academic performance. Moreover, self-efficacy emerged as a partial mediator for the relationship between conscientiousness and performance.