Studies indicate the COVID-19 pandemic has resulted in rises in adolescent mental health symptoms globally, although the impact of the pandemic on subjective wellbeing is under-researched in this population. Psychological capital (PsyCap), a cluster of four positive psychological constructs comprising hope, efficacy, resilience and optimism (HERO), has demonstrated preventative and promotive qualities on mental health symptoms and subjective wellbeing outcomes with adult populations (employees, university students). However, PsyCap’s influence on these outcomes in young people is unclear. The present exploratory study investigated changes in self-reported anxiety and depressive symptoms (measured via the RCADS-SV) and subjective wellbeing (measured by the Flourishing Scale) from pre-pandemic levels to 3 months into the pandemic and explored gender differences at each time point in a sample of Australian Year 10 students (N = 56, Mage = 14.93 years, SD = 0.50, 51.8% male). The longitudinal predictive role of baseline PsyCap on follow-up assessments of anxiety symptoms, depressive symptoms and flourishing were also examined. There were no significant changes in levels of anxiety and depressive symptoms between the timepoints, but flourishing significantly declined from T1 to T2. Baseline PsyCap was not a significant predictor of T2 anxiety and depressive symptoms but was a significant predictor of T2 flourishing. Further, different baseline HERO constructs predicted T2 mental health symptoms and flourishing. Future larger studies building on the current preliminary findings investigating the roles of student PsyCap, mental health and subjective wellbeing are warranted to better understand these constructs in the COVID-19 era and beyond.