Objective: This study compares the mental health of single parents relative to partnered parents and assesses the contribution of the social and demographic factors to this difference, examining the gender difference in it. Methods: We analyzed 12,024 single and partnered subjects, aged 30–59 years, living with children, aged 0–19 years, drawn from the 4th, 5th, and 6th Korea National Health and Nutrition Examination Survey (KNHANES) dataset in South Korea conducted from 2007–2013. Mental health was evaluated by self-reported questionnaires including depressive mood for recent two weeks, presence of suicidal ideation, and the Korean version of the Alcohol Use Disorder Identification Test. Covariates included age, physical illness, socioeconomic status (family income, recipient of national basic livelihood guarantees, educational level, house ownership, job, and residential area), family structure, and support (co-residence of another adult). Multiple logistic regression was carried out and the explained fractions of each covariate was calculated. Results: Single parents had significantly poorer mental health than their partnered counterparts, with odds ratio (OR) of 2.02 (95% confidence interval (CI) 1.56–2.63) for depressive symptoms, 1.69 (95% CI 1.27–2.25) for suicidal ideation, and 1.74 (95% CI 1.38–2.20) for any of the three mental health statuses (suspicious depression, suicidal ideation, and alcohol dependence) after controlling for the covariates. The odds of depressive symptoms (OR = 3.13, 95% CI 2.50–3.93) and suicidal ideation (OR = 2.50, 95% CI 1.97–3.17) among both single fathers and mothers were higher than partnered parents. However, the odds of alcohol dependence were 3.6 times higher among single mothers than partnered mothers (OR = 3.58, 95% CI 1.81–7.08) and were 1.4 times greater among single fathers than partnered fathers (OR = 1.35, 95% CI 0.81–2.25). Socio-economic status explained more than 50% (except for substance use disorders) of the poorer mental health in single parents. These results were more remarkable for single fathers than for single mothers except for alcohol dependence. However, physical illness, family structure, and support made only minor contributions to single parents’ mental health. Conclusions: This study demonstrates that single parents have poorer mental health than partnered parents. Although lower SES is an important factor explaining poorer mental health in single parents, there are other factors we cannot explain about their poor mental health. Therefore, we should pay proper regard to identifying other factors affecting mental health and to establishing policies to support single parents.