Background: One in three adolescents with chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) have mental health problems. Multi-informant perspectives are key to psychological assessment. Understanding parent-child agreement is crucial to accurate diagnosis, particularly where severe fatigue limits self-report. Methods: Agreement on the revised children’s anxiety and depression scale (RCADs) was assessed between parents and children with CFS/ME ( n = 93) using Bland-Altman plots, cross tabulations and regression analyses. Results: Diagnostic thresholds were met more frequently based on child-report. Parent- and child-report had similar sensitivity and specificity on RCADS compared to gold-standard diagnostic interviews. Regression analysis found similar accuracy between both reports. For anxiety diagnoses, odds ratio (OR) for child-report was 1.10 (CI = 1.06–1.14), and 1.10 (CI = 1.05–1.14) for parent-report. For depression, OR for child report was 1.26 (CI = 1.11–1.43), while for parent-report is was 1.25 (CI = 1.10–1.41). For total score, OR for child-report was 1.10 (CI = 1.05–1.13) while OR for parent-report was 1.09 (CI = 1.05–1.13). Conclusions: Reasonable agreement was observed between parent- and child-report of mental health symptoms in paediatric CFS/ME. While parent-report can facilitate psychological evaluation in CFS/ME, this is not a substitute for a child’s own report.