This systematic review was conducted to determine the prevalence of mental disorders among children and adults in Uganda. A comprehensive systematic search for relevant studies reporting prevalence of mental disorders in children or adults in Uganda was conducted in PubMed, Embase, PsycINFO, Scopus, Web of Science databases and grey literature sources. Study was eligible if, validated instrument based on the International Classification of Diseases or Diagnostic and Statistical Manual of Mental Disorders criteria to assess a mental disorder was used. Eligible studies were critically appraised, prevalence data extracted and pooled using the random-effects model. Certainty in the pooled prevalence estimates was evaluated using the Grading of Recommendation, Assessment, Development and Evaluation approach. A total of 632 records were obtained, of which 26 articles from 24 studies conducted in Uganda were included in the review. Overall and with moderate level of certainty, the prevalence of any mental disorder in Uganda was 22.9% (95% C.I 11.0% - 34.9%) in children and 24.2% (95% C.I 19.8% - 28.6%) in adults. Prevalence of anxiety disorders was 14.4% (95% C.I 4.9% - 24.0%) in children and 20.2% (95% C.I 14.5% - 25.9%) in adults. The prevalence of current depressive disorders was 22.2% (95% C.I 9.2% - 35.2%) in children and 21.2% (95% C.I 16.8% - 25.6%) in adults. Eating disorder and psychotic syndrome disorder were also reported. Our findings suggest that depression and anxiety disorders are common mental disorders in Uganda, affecting approximately one in four persons. The findings provide essential insights for health service planning, clinical practice, and future epidemiological research in Uganda.