Objective The aims of this study were to understand the mental health of single fathers relative to married fathers as the population of single fathers continues to increase and to use the resultant data for the establishment of public health policies. Methods We evaluated the mental health of 58 single fathers and 256 married fathers living in an urban community in South Korea. Self-reported questionnaires including the Global Assessment of Recent Stress, Center for Epidemiologic Studies-Depression, Scale for Suicidal Ideation, the Korean version of the Alcohol Use Disorder Identification Test, and the World Health Organization Quality of Life Assessment Instrument were used for evaluation. The mental health scale scores and the prevalence of mental health problems were compared between the single and the married fathers. We also assessed the factors associated with poor QOL, depressive symptoms, and severe stress among single fathers. Results The single fathers had poorer quality of life (OR 7.30, 95% CI 2.82-18.74), more depressive symptoms (OR 3.85, 95% CI 1.29-11.45), and more stress (OR 3.36, 95% CI 1.25-8.98) than did the married fathers even after controlling for socio-demographic factors. Among the single fathers, poor socioeconomic conditions, such as no house ownership, manual occupations, having two or more children, and having a youngest child in elementary school or middle school, were significantly associated with poorer mental health. Conclusions This study demonstrated that single fathers have poorer mental health than do married fathers. Single parenthood was significantly associated with their mental health. Therefore, we should attempt to provide practical support to reduce the social burden and offer earlier psychological interventions to reduce distress in single fathers.