Background: Children with mild/moderate intellectual disabilities are at greater risk for mental health problems, with socio-economic factors and adversity partly accounting for this. Fewer data are available for adolescents. Methods: Secondary analysis was undertaken of the "Next Steps" annual panel study following a cohort through adolescence into adulthood containing self-report mental health data up to age 16/17. Participants with mild/moderate intellectual disabilities were identified through data linkage with educational records. Results: Adolescents with mild/moderate intellectual disabilities were more likely than non-disabled peers to experience socio-economic disadvantage and bullying. Incidence rates of mental health problems were generally not significantly different between adolescents with and without intellectual disabilities. Conclusions: These findings are consistent with higher rates of persistent mental health problems beginning earlier among children with intellectual disabilities. Greater attention needs to be paid to the timecourse of mental health problems, and the impact of socio-economic factors, family and peers on mental health.