BACKGROUND People with a history of complex traumatic events typically experience trauma and stressor disorders and additional mental comorbidities. It is not known if existing evidence-based treatments are effective and acceptable for this group of people. OBJECTIVE To identify candidate psychological and non-pharmacological treatments for future research. DESIGN Mixed-methods systematic review. PARTICIPANTS Adults aged ≥ 18 years with a history of complex traumatic events. INTERVENTIONS Psychological interventions versus control or active control; pharmacological interventions versus placebo. MAIN OUTCOME MEASURES Post-traumatic stress disorder symptoms, common mental health problems and attrition. DATA SOURCES Cumulative Index to Nursing and Allied Health Literature (CINAHL) (1937 onwards); Cochrane Central Register of Controlled Trials (CENTRAL) (from inception); EMBASE (1974 to 2017 week 16); International Pharmaceutical Abstracts (1970 onwards); MEDLINE and MEDLINE Epub Ahead of Print and In-Process & Other Non-Indexed Citations (1946 to present); Published International Literature on Traumatic Stress (PILOTS) (1987 onwards); PsycINFO (1806 to April week 2 2017); and Science Citation Index (1900 onwards). Searches were conducted between April and August 2017. REVIEW METHODS Eligible studies were singly screened and disagreements were resolved at consensus meetings. The risk of bias was assessed using the Cochrane risk-of-bias tool and a bespoke version of a quality appraisal checklist used by the National Institute for Health and Care Excellence. A meta-analysis was conducted across all populations for each intervention category and for population subgroups. Moderators of effectiveness were assessed using metaregression and a component network meta-analysis. A qualitative synthesis was undertaken to summarise the acceptability of interventions with the relevance of findings assessed by the GRADE-CERQual checklist. RESULTS One hundred and four randomised controlled trials and nine non-randomised controlled trials were included. For the qualitative acceptability review, 4324 records were identified and nine studies were included. The population subgroups were veterans, childhood sexual abuse victims, war affected, refugees and domestic violence victims. Psychological interventions were superior to the control post treatment for reducing post-traumatic stress disorder symptoms (standardised mean difference -0.90, 95% confidence interval -1.14 to -0.66; number of trials = 39) and also for associated symptoms of depression, but not anxiety. Trauma-focused therapies were the most effective interventions across all populations for post-traumatic stress disorder and depression. Multicomponent and trauma-focused interventions were effective for negative self-concept. Phase-based approaches were also superior to the control for post-traumatic stress disorder and depression and showed the most benefit for managing emotional dysregulation and interpersonal problems. Only antipsychotic medication was effective for reducing post-traumatic stress disorder symptoms; medications were not effective for mental comorbidities. Eight qualitative studies were included. Interventions were more acceptable if service users could identify benefits and if they were delivered in ways that accommodated their personal and social needs. LIMITATIONS Assessments about long-term effectiveness of interventions were not possible. Studies that included outcomes related to comorbid psychiatric states, such as borderline personality disorder, and populations from prisons and humanitarian crises were under-represented. CONCLUSIONS Evidence-based psychological interventions are effective and acceptable post treatment for reducing post-traumatic stress disorder symptoms and depression and anxiety in people with complex trauma. These interventions were less effective in veterans and had less of an impact on symptoms associated with complex post-traumatic stress disorder. FUTURE WORK Definitive trials of phase-based versus non-phase-based interventions with long-term follow-up for post-traumatic stress disorder and associated mental comorbidities. STUDY REGISTRATION This study is registered as PROSPERO CRD42017055523. FUNDING This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 24, No. 43. See the NIHR Journals Library website for further project information.