Key PointsQuestionAre adult survivors of childhood cancer at risk for new-onset neurocognitive impairments that outpace those associated with typical aging?FindingsIn this cohort study of 2375 adult survivors of childhood cancer and their sibling controls, new-onset memory impairment emerged more often in survivors decades after cancer diagnosis and treatment. The increased risk was associated with cancer treatment, modifiable health behaviors, and chronic health conditions.MeaningThese findings suggest that adult survivors of childhood cancer are at elevated risk for new-onset neurocognitive impairments as they age and that such new-onset impairment may be an indicator of future neurocognitive decline and possibly dementia.