From a positive psychology perspective, adopting a growth mindset in language learning cultivates positive learning goals and sustains motivational drive. However, the extent to which these motivational benefits translate into tangible language learning outcomes remains controversial. Drawing from the language learning motivational process model, this study investigates the predictive power of growth language mindset on second language (L2) achievement and the mediating role of autonomous motivation and engagement. High-school English language learners in China (n = 486) completed mindset, motivation, and engagement questionnaires, and we retrieved their subsequent English test scores. The results suggested that growth mindset positively, albeit weakly, predicted subsequent L2 achievement (r = .17). Further, autonomous motivation and engagement sequentially mediated the link between growth mindset and L2 achievement. Engagement also acted as an independent mediator (although this was not the case for autonomous motivation). The study provides theoretical insights into the mechanisms through which growth mindset can positively impact L2 learning, and identifies practical pedagogical implications for L2 classrooms.