Mental toughness has been associated with optimal performance across diverse contexts including academic achievement. MT is positively associated with subclinical narcissism. Cross-sectional research reported that high narcissism may contribute indirectly to enhanced positive outcomes, through MT. This study is the first to explore longitudinally the development of the association between MT, narcissism and achievement in a sample of adolescents. MT correlated positively with narcissism and predicted a small percentage of the variation in school achievement. Narcissism did not correlate significantly with school achievement. However, subclinical narcissism exerted a significant positive indirect effect on school achievement through MT. The findings suggest that the relationship between narcissism and MT could be one of the non-cognitive mechanisms that underlie individual variation in school achievement.