Capitalizing on the implementation of a writing assessment initiative implemented at a major Chilean university, we test how predictive writing is of subsequent academic achievement. First, using a multilevel analytic approach (n = 2597), the study shows that, after controlling for socio-demographic variables and the university admission tests, writing skills significantly predict first-year university grades. Second, using information about the performance of students during their first eight semesters in the university (n = 1616), a longitudinal hierarchical analysis showed that writing remains a significant predictor of university grades over time, also after controlling socio-demographic variables and university admissions tests. Moreover, language skills retain or improve their predictive role over time, whereas mathematics skills seem to decrease in their importance. Our results show that writing, and the cognitive skills involved in writing, play a critical role in advanced stages of academic training, consequently offering additional support for the consideration of this ability for university admission purposes.