Background Adverse childhood experiences (ACEs) can increase risks of health-harming behaviours and poor health throughout life. While increases in risk may be affected by resilience resources such as supportive childhood relationships, to date few studies have explored these effects. Methods We combined data from cross-sectional ACE studies among young adults (n = 14 661) in educational institutions in 10 European countries. Nine ACE types, childhood relationships and six health outcomes (early alcohol initiation, problem alcohol use, smoking, drug use, therapy, suicide attempt) were explored. Multivariate modelling estimated relationships between ACE counts, supportive childhood relationships and health outcomes. Results Almost half (46.2%) of participants reported ≥1 ACE and 5.6% reported ≥4 ACEs. Risks of all outcomes increased with ACE count. In individuals with ≥4 ACEs (vs. 0 ACEs), adjusted odds ratios ranged from 2.01 (95% CIs: 1.70–2.38) for smoking to 17.68 (95% CIs: 12.93–24.17) for suicide attempt. Supportive childhood relationships were independently associated with moderating risks of smoking, problem alcohol use, therapy and suicide attempt. In those with ≥4 ACEs, adjusted proportions reporting suicide attempt reduced from 23% with low supportive childhood relationships to 13% with higher support. Equivalent reductions were 25% to 20% for therapy, 23% to 17% for problem drinking and 34% to 32% for smoking. Conclusions ACEs are strongly associated with substance use and mental illness. Harmful relationships are moderated by resilience factors such as supportive childhood relationships. Whilst ACEs continue to affect many children, better prevention measures and interventions that enhance resilience to the life-long impacts of toxic childhood stress are required.