Concerns about mental health difficulties and reduced wellbeing in adolescents are on the rise in public discourses and the media. This study utilised data from the Millennium Cohort Study (Waves 6 and 7) to examine 17- year- olds' mental health and wellbeing in relation to gender and personality traits, and longitudinal differences in social and emotional difficulties and prosocial behaviour from mid to late adolescence. Across measures of mental health and wellbeing, the findings showed that adolescent girls fared much worse than boys, particularly in reporting psychological distress and self-harm. Longitudinal analyses showed that as young women moved from mid to late adolescence, they scored higher in psychological distress and social, emotional and behavioural difficulties. Also, compared to boys, girls rated themselves higher on neuroticism, a personality trait, which emerged as a strong predictor of psychological distress. The findings from this study have implications for young people's mental health and wellbeing and gender inequality. Given the current policy concerns about young people's mental health difficulties and self-harm, this study is hoped to contribute to an informed debate about wellbeing through gender and personality lenses while raising questions about current understandings of wellbeing and education and public health policy developments.