Adolescence marks a critical transition period, with significant mental health challenges including anxiety and depression symptoms that affect long-term happiness. There has been a lack of research exploring the factors mediating adolescent happiness.

To investigate the mediating effects of anxiety and depression on adolescent happiness, as well as the contributions of sociodemographic factors.

We recruited 392 adolescents. Anxiety symptoms, depression symptoms and happiness were assessed by the seven-item Generalized Anxiety Disorder scale, nine-item Patient Health Questionnaire and single-item happiness scale, respectively. Self-administered questionnaires were used to collect sociodemographic information.

Spearman correlation analysis showed significant negative correlations of happiness with anxiety (r = −0.37, P < 0.0001) and depression (r = −0.47, P < 0.0001). Positive predictors of happiness included quality of parents’ marriage (β = 0.12, P = 0.006), regular physical exercise (β = 0.13, P = 0.006) and regular diet (β = 0.10, P = 0.03). Mediation analysis indicated that depressive symptoms (estimate = 0.50, 95% CI: 0.25 to 0.80) and anxiety symptoms (estimate = 0.32, 95% CI: 0.12 to 0.57) partially mediated the relationship between regular exercise and happiness, whereas depressive symptoms completely mediated the relationship between anxiety symptoms and happiness (estimate = −0.14, 95% CI: −0.20 to −0.08).

The findings of this study highlight the intricate interplay of mental health issues, lifestyle factors and adolescent happiness and emphasise the need for comprehensive interventions focusing on enhancing physical activity and addressing psychological health to foster happiness among adolescents.