Background The mental health outcomes of military personnel deployed on peacekeeping missions have been relatively neglected in the military mental health literature. Aims To assess the mental health impacts of peacekeeping deployments. Method In total, 1025 Australian peacekeepers were assessed for current and lifetime psychiatric diagnoses, service history and exposure to potentially traumatic events (PTEs). A matched Australian community sample was used as a comparator. Univariate and regression analyses were conducted to explore predictors of psychiatric diagnosis. Results Peacekeepers had significantly higher 12-month prevalence of post-traumatic stress disorder (16.8%), major depressive episode (7%), generalised anxiety disorder (4.7%), alcohol misuse (12%), alcohol dependence (11.3%) and suicidal ideation (10.7%) when compared with the civilian comparator. The presence of these psychiatric disorders was most strongly and consistently associated with exposure to PTEs. Conclusions Veteran peacekeepers had significant levels of psychiatric morbidity. Their needs, alongside those of combat veterans, should be recognised within military mental health initiatives.