Background: Many empirical investigations focus on how personality traits and academic motivation are related to academic achievement. Regarding the personality traits described in the five-factor model, prior research has shown associations between openness to experience and language achievement in particular. Following the principle of trait activation, associations with intrinsic value can explain these domain-specific relationships of openness with achievement. Aims: This study investigated whether intrinsic value mediates the associations of openness to experience with language achievement to understand these associations more closely. Based on the principle of trait activation, we combined openness to experience with intrinsic value as a motivational construct more proximal to behaviour. Samples: We capitalized on two large-scale longitudinal data sets (N = 1994; M = 16.8 years; 51% female; and N = 2722; M = 17.34 years; 58% female) of German-speaking students in upper secondary education with two points of measurement, respectively. Methods: We assessed achievement using report card grades and standardized test scores, capturing three language subskills: reading and listening, measured by standardized tests, and writing competencies, measured by essay assignments. We conducted mediation analysis using bootstrapped confidence intervals and robust standard errors to account for dependencies in the data due to students clustered within classrooms. Results and Conclusions: Overall, our results show that intrinsic value mediated the relationship between openness and achievement in English as a foreign language. The results remained stable when controlling for prior achievement, predicting achievement gains. Our investigation provides empirical insights into the manifestations of personality in academic contexts. We discuss the limitations of our approach, with a focus on the use of a mediation analysis in this research design.