Background Sri Lanka has experienced large-scale forced internal displacement of people due to conflict. The impact of prolonged displacement and resettlement on mental health ans resilience are poorly understood. Objectives: Key objectives were to describe the prevalence, incidence and maintenance of common mental disorder (CMD) and post-traumatic stress disorder (PTSD) in an adult sample internally displaced because of conflict since 1991 and currently considering returning to areas of origin. Levels of personal resilience, and social network/social support were also investigated in relation to CMD. Methods: A cross-disciplinary survey sample of 450 IDPs living in displacement was recruited and followed one year later, supplemented at follow-up by an additionally recruited sample of 228 IDPs who had return-migrated. CMD was measured by the Patient Health Questionnaire and PTSD by the Composite International Diagnostic Interview K-section. Resilience was measured with the Resilience Scale-14 item version. Social support and social networks were measured using the Multi-Dimensional Social Support and Lubben Social Network scales. Results: The baseline prevalence of any CMD was 18.8%, PTSD prevalence was 2.4%. The CMD prevalence had reduced to 8.6% at folloow-up in those remaining in displacement, and was 10.3% in return migrants. PTSD prevalence were 0.3% and 1.6% respectively; unemployment, widowed or divorced status, female gender and food insecurity were independently associated with CMD. The mean resilience scores were 80.2 at baseline and 84.9 at follow-up. At both time points, lower resilience was interdependently associated with food insecurity, lower social support availability and social isolation, and there were significant associations with CMD in unadjusted analyses, but only independent at baseline. Conclusions: Policy implications on providing post-conflict mental health care to IDPs in Sri Lanka stem from findings, providing important insights into interventions that can reduce the risk of mental disorders and promote resilience.