Objectives: To examine the extent to which social support and socio-economic status (SES) in adulthood mediate the impact of interpersonal adversity in childhood on adult mental health using large-scale population data in Japan. Study design: Cross-sectional survey. Methods: Data were derived from the Japanese Study of Stratification, Health, Income and Neighbourhood, which was conducted from October 2010 to February 2011 in four municipalities in and around the Tokyo metropolitan area. Survey participants were community residents aged 25-50 years who were selected at random from voter registration lists. The total sample size was 3292. The self-reported experience of parental maltreatment (physical abuse and/or neglect) and bullying in school and their impacts on adult mental health (in terms of K6=5+, K6=13+ and suicide ideation) were examined using multivariate logistic models. Results: Interpersonal adversity in childhood has a negative impact on adult mental health even after controlling for childhood SES. For example, the odds ratio for K6=5+, responding to parental maltreatment, was 2.64 (95% confidence interval 2.04-3.41). Perceived social support and adult SES mediated the impact of interpersonal adversity in childhood, but a substantial proportion of the impact was unexplained by their mediating effects; social support and adult SES only mediated 11-24% and 6-12%, respectively. It was also found that social support and adult SES (except educational attainment) did not moderate the negative impact of interpersonal adversity in childhood. Conclusions: This analysis highlighted that the impact of interpersonal adversity in childhood is relatively independent of social support and SES in adulthood. This result has clear policy implications; more focus should be placed on policies that aim to reduce incidents of childhood maltreatment and bullying per se, both of which have a long-lasting direct impact on mental health. © 2013 The Royal Society for Public Health.