Survivors of pediatric brain tumor (BT) are known to be at risk for developing cognitive and psychosocial late effects. Young age at treatment (≤6 years) is typically considered to put patients at increased risk. However, there is limited research specifically exploring functioning in these young patients. Cognitive and psychosocial data were retrospectively abstracted from medical charts for 79 young patients (54.4% male) treated for BT with a variety of treatment modalities (e.g., surgery, radiation therapy, chemotherapy). Children were clinically assessed at 4.52 years of age (range = 1.48–5.98) and most were off-therapy (74.4%). Mean performances on developmental (68.3 ± 10.02), cognitive (88.09 ± 18.38), and pre-academic (86.84 ± 19.75) measures were all below average. Parent report of adaptive functioning was also below average (82.10 ± 16.21), but psychosocial functioning was generally within normal limits. Most patients had impaired functioning (scores <10th percentile) in at least one domain assessed. Exploratory analyses revealed that many patients (27.3–60.6%) exhibited a significant discrepancy between domains of cognitive functioning (e.g., verbal and spatial). Young children treated for BT experienced high rates of impairment in cognitive, pre-academic, and adaptive domains. Future work is needed to focus on serial longitudinal assessment of these young patients, as well as dedicated intervention and prevention efforts.