Background There are few studies of mental ill health among young people in developing countries. Aims To measure the prevalence of common mental disorders among low-income young people in the city of Cali, Colombia and to examine associations with violence and social capital. Method The Self-Reporting Questionnaire was administered to 1057 young people aged 15–25 years. Social capital, violence, alcoholism and socio-demographic variables were also measured. Results We found 255 young people (24%) with common mental disorders. Being a woman, having limited education and experiencing high levels of violence were the main risk factors for common mental disorders. Social capital did not emerge as a risk factor. Conclusions A large burden of mental ill health among young people was found; this requires urgent interventions and more research on the mechanisms which link mental health and violence.