The mental health of Sri Lankan adolescents is of growing concern, given the decades of internal conflict and socio-political instability in Sri Lanka. This aims were to examine the prevalence and determinants of symptoms of common mental health problems (MHP) experienced by school-going adolescents in Sri Lanka. A cross-sectional survey was conducted among school-going adolescents in grades 10–12/13 from seven schools in Gampaha District, Sri Lanka. Depressive/psychological distress symptoms measured using the PHQ-9 /K10, were analysed using mean scale scoring. Psychosocial determinants were measured using JVQ/PBI/AESI/study-specific questions. Associations between MHPs and psychosocial determinants were examined using multiple linear regression models. 24.11% of 1,045 adolescents who completed the surveys reported clinically significant symptoms of depression, and 60.10% reported psychological distress. Higher age, being female, lesser physical activity, smoking, daily social media use, violent victimisation, not living with both birth parents, having ≥2 siblings, low maternal/paternal education, having an overprotective paternal figure, increased academic stress and rural living were associated with higher MHPs. We identified a high prevalence of MHPs among Sri Lankan adolescents, which was multifactorially determined. Modifiable risk factors addressed through public health policies, research and programmes, as well as less-modifiable risk factors addressed through national-level policy changes, are all essential to addressing mental health burdens in this population.