Mitotane is used as a post-operative adjuvant treatment for patients with adrenocortical carcinoma. Monitoring of plasma mitotane concentrations is recommended, but we do not know what impact target concentrations have on patient outcome. To answer this question, we retrospectively analyzed patient records in the Lysosafe Online® database (HRA Pharma, France) for patients who were treated for ≥6 months and who had ≥3 measurements of plasma mitotane levels during follow-ups at 11 tertiary centers in Italy from 2005 to 2017. We identified 110 patients treated with adjuvant mitotane for a median of 46 months (IQR, interquartile range, 28-62) with a median maintenance dose of 2.0 g/day (IQR 1.5-2.5). Achievement of target mitotane concentrations (≥14 mg/L) required a median of 8 months (IQR 5-19). Female sex was associated inversely with the dose, while body mass index (BMI) was correlated positively. Multivariate analysis showed that the Ki67 index and time to achieve the target range of plasma mitotane were independent predictors of recurrence-free survival (RFS). In a separate multivariate model, considering only the maintenance phase (month 7 to month 36, M7-M36) of treatment, the time in the target range of plasma mitotane was associated with a significantly lower risk of recurrence (Hazard Ratio, HR = 0.93; 0.88-0.98, p < 0.01). The prognostic implications of the time in target range and the time needed to reach target mitotane concentrations support the use of mitotane monitoring and may inform practice.