To develop and validate a prediction model for disability among young patients with acquired brain injury (ABI) after the acute phase. Within a nationwide cohort of 446 15–30-year-old ABI-patients, we predicted disability in terms of Glasgow Outcome Scale – Extended (GOS-E) <7 12 months after baseline assessment in outpatient neurorehabilitation clinics. We studied 22 potential predictors covering demographic and medical factors, clinical tests, and self-reported fatigue and alcohol/drug consumption. The model was developed using multivariable logistic regression analysis and validated by 5-fold cross-validation and geographical validation. The model’s performance was assessed by receiver operating characteristic curves and calibration plots. Baseline assessment took place a median of 12 months post-ABI. Low GOS-E (range 1–8 (best)) and Functional Independence Measure (range 18–126 (best)) along with high mental fatigue (range 4–20 (worst)) predicted disability. The model showed high validity and performance with an area under the curve of 0.82 (95% confidence interval (CI) 0.77, 0.87) in the cross-validation and 0.81 (95% CI 0.73, 0.88) in the geographical validation. We developed and validated a parsimonious model which effectively predicted disability. The model may be useful to guide decision-making in outpatient neurorehabilitation clinics treating young patients with ABI.