Adjuvant treatment with mitotane and chemotherapy is recommended for paediatric advanced and metastatic adrenocortical carcinoma (ACC). Yet, questions on the indication, dosage, and length of therapy are unanswered. Data from the German Paediatric Oncology Haematology-Malignant Endocrine Tumour studies were analysed retrospectively for patients receiving mitotane during first- and/or second-line therapy. Forty-three patients were identified (median age: 7.5 years (range: 0.2–17.8); 29 female) with median follow-up of 2.2 years (range: 0.04–12.71). Three-year overall (OS) and progression-free survival (PFS) were 44.9% and 28.5%, respectively. Eleven of 43 patients received mitotane as neoadjuvant treatment, and 4/11 tumours reached partial remission (PR). Twenty-seven of 43 patients received mitotane combined with chemotherapy in an adjuvant setting resulting in PR of measurable target lesions in 5/13 patients. Metastatic disease (hazard ratio (HR): 3.2; 95% CI: 1.2–18.6; P = 0.018), duration of mitotane treatment <9 months (HR: 5.6; 95% CI: 1.9–16.9; P = 0.002), and not achieving drug target range (TR) (HR: 28.5; 95% CI: 5.4–150.3; P < 0.001) significantly impacted as negative prognostic factors upon PFS and OS (metastatic disease: HR: 4.9; 95% CI: 1.6–15.5; P = 0.006; duration of mitotane treatment: HR: 7.0: 95% CI 1.9–26.0; P = 0.004; TR not reached: HR: 13.5; 95% CI 3.6–50.3; P < 0.001). Cox regression determined the risk of event decreasing by 10.4% for each month of mitotane treatment (P = 0.015). Re-treatment with mitotane after first-line treatment proved ineffective. The duration of mitotane treatment and reaching mitotane TR significantly impacted survival. Improving the efficacy of mitotane, including appropriate indications, needs to be evaluated in prospective randomized trials.