With a post-pandemic churn gaining steam, foreign language (FL) learning and teaching are becoming increasingly challenging. This study investigated young FL learners' language mindsets, negative emotions, emotion regulation, and their relations in online FL classes during the COVID-19 pandemic. An altogether 640 young Chinese FL learners aged between 11 and 19 from a snowball sampling were enrolled and surveyed. Three major findings were obtained. First, the participants overall reported a higher growth mindset toward their FL learning and were experiencing a medium magnitude of negative emotions in online FL classes, with the feelings of shame, anxiety, and boredom being strongest, followed by anger and hopelessness. They demonstrated a relatively high level of emotion regulation when learning the FL online. Second, the growth language mindset (significantly) negatively and the fixed language mindset significantly and positively predicted the participants' negative emotions in online classes. Third, a 'growth language mindset -- emotion regulation -- emotion' pattern emerged in the data, with the emotion regulation mediating the predictive effects of the participants' growth language mindset on negative emotions (boredom and anger) in online FL learning. Pedagogical implications and suggestions for further research were also discussed.