Aim: To investigate the relationship between growth, body composition, and the extent of brain lesion measured using structural magnetic resonance imaging (MRI) in children with cerebral palsy (CP). Method: This prospective population-based cohort study recorded 359 assessments from 124 children with CP aged 18 months to 13 years (38% female, Gross Motor Function Classification System [GMFCS] levels I = 50, II = 24, III = 17, IV = 12, and V = 21). A neurologist assessed the extent of the brain lesion using a validated semi-quantitative scale (global, basal ganglia/brainstem, hemispheric and corpus callosum scores). Height (HTZ), weight (WTZ), and head circumference (HDZ) z-scores were calculated. The Fat Mass Index (FMI) and Fat-Free Mass Index (FFMI) were determined using a deuterium dilution technique, bioelectrical impedance or dual-energy X-ray absorptiometry, and height. Data were analysed using mixed-effects linear regression. Results: Greater global (β = −0.04, 95% confidence interval [CI] = −0.07 to −0.02), basal ganglia/brainstem (β = −0.06, 95% CI = −0.11 to −0.02), corpus callosum (β = −0.27, 95% CI = −0.27 to −0.12), and hemispheric (β = −0.08, 95% CI = −0.12 to −0.04) scores were associated with lower HTZ. Greater global (β = −0.03,95% CI = −0.06 to −0.01) and corpus callosum (β = −0.23, 95% CI = −0.40 to −0.06) scores were associated with lower WTZ. A greater hemispheric score (β = −0.06, 95% CI = −0.119 to −0.001) was associated with lower HDZ. Semi-quantitative MRI scores were not associated with FMI or FFMI. Interpretation: Greater extent of the brain lesion was significantly associated with lower HDZ, HTZ, and WTZ but not body composition in children with CP aged 18 months to 13 years.