This study aimed to assess the prevalence of common mental disorders (CMD) and their association with quality of life among young adults in Pelotas, Rio Grande do Sul State, Brazil. This was a cross-sectional population-based study of young people 18 to 24 years of age. Sampling used simple cluster selection. Investigation of CMD used the Self-Report Questionnaire (SRQ-20), while quality of life was measured with the Medical Outcomes Survey Short-Form General Health Survey (SF-36). Prevalence of CMD in the study sample was 24.5% (N = 382), and was positively associated with: female gender, lower socioeconomic status (classes D or E), not studying, not working, alcohol consumption, smoking, and substance abuse. Young adults with CMD had lower mean scores on the SF-36 in all the assessed quality of life domains. Measures to prevent CMD are necessary to provide better quality of life for this age group.