The COVID-19 pandemic disrupted opportunities for adolescents to progress through a typical developmental trajectory of adjustment and self-regulation. Adolescents across many contexts have shown an increase in adjustment difficulties during the pandemic compared to pre-pandemic levels. Utilizing data collected from 830 Italian adolescents from one time-point just prior to when pandemic restrictions were enacted, and then at two additional time-points 12 and 24 months later, we examined trajectories of internalizing symptoms, regulatory self-efficacy for managing negative emotions, and the time-varying impact of COVID-related stress and social support. Latent Growth Curve Analyses (LGCAs) revealed that COVID stress predicted increased internalizing symptoms and decreased regulatory self-efficacy beyond what was estimated by the developmental trajectory. Further, at Time 3, the impact of perceived COVID stress on internalizing symptoms and regulatory self-efficacy was stronger than at Time 2. Co-development results were similar for males and females. There was no time-varying impact of social support on the co-development of internalizing symptoms and regulatory self-efficacy. The findings are important for informing interventions to strengthen coping strategies for adolescents during stressful community-wide events.