The majority of previous research on the language mindset (LM) and academic engagement (AEG) of learners was dominated by the focus on traditional in-person classrooms, whereas less is known about the influence of the beliefs of learners toward language learning on their AEG, which comprises behavioral, emotional, cognitive, and agentic engagement in the online setting. Toward this end, this study sought to investigate the influence of the LM of learners on their AEG in online classrooms and if and how this link is mediated by achievement emotions (e.g., anxiety and enjoyment). The results of partial least squares path modeling revealed that the growth LM positively predicted all components of AEG. While enjoyment functioned only as a mediator, which linked growth LM to emotional engagement, anxiety linked growth LM to all subcomponents of AEG. Moreover, out of the two emotions examined by this study, anxiety negatively predicted the four components of AEG, whereas enjoyment only influenced emotional engagement. On the basis of these findings, the study presented practical implications and future research directions.