Background: Excessive worry during adolescence can significantly impact mental health. Understanding adolescent concerns may help inform mental health early intervention strategies. Objective: This study aimed to identify frequent concerns among Australian secondary school students, exploring individual and demographic differences. Whether adolescents' most frequently reported concern was associated with mental health and wellbeing was also investigated. Methods: A total of N = 4086 adolescents (Mage = 13.92) participated in an online survey, reporting their top concerns alongside demographic characteristics, mental health, and wellbeing. Data were analysed using both qualitative and quantitative approaches. Results: Thematic analysis identified 11 different themes of adolescent concerns. A frequency analysis showed concerns relating to 'School and Academics' were most common (24.52% of all responses), consistent across females, males, school location (regional vs metropolitan areas), and socioeconomic background. Sexuality and gender diverse adolescents more frequently reported concerns about 'Mental Health and Wellbeing' and 'Social Relationships'. Linear mixed models found 'School and Academic' concerns were associated with lower symptoms of depression (p < 0.001, d = 0.16) and anxiety (p < 0.001, d = 0.19) and higher wellbeing (p = 0.03, d = 0.07) compared to all other concerns. Conclusion: 'School and Academic' concerns were most common, however not associated with poorer mental health or wellbeing. Sexuality and gender diverse adolescents were more likely to report concerns regarding 'Mental Health and Wellbeing' and 'Social Relationships'. Efforts aimed solely at reducing academic stress may not be the most effective approach to improving adolescent mental health. Longitudinal data into how concerns evolve over time could provide a nuanced understanding of their relationship with future mental health challenges.