High-dose radiation in childhood such as is used in radiation therapy causes cognitive decline, but there is insufficient research on the cognitive effects of low- to medium-dose radiation. We aimed to reveal the association between atomic bomb radiation exposure in childhood and late-life neurocognitive function. In 2011 and 2013, we mailed the Neurocognitive Questionnaire (NCQ) to subjects who were 12 years old or younger at the time of the atomic bombing. We converted the four NCQ subscales (metacognition, emotional regulation, motivation/organization, and processing speed) to T scores and defined the highest 10% of the controls (exposure dose < 5 mGy) as impaired. We used a generalized linear mixed model to evaluate the effect of radiation exposure on T scores and percentage impaired. We enrolled 859 participants. At the time of the bombing, the mean (SD) age was 6.7 (3.8) years for the control (N = 390) and 6.1 (3.8) years for the exposed (N = 469). At the time of replying to the questionnaire, the mean age of all the participants was 73.7 (3.8) years of age. After adjusting for cofactors, older age was related to the decline of all neurocognitive subscales. Sex and education level had relevance to some of the subscales. For neurocognitive function, exposure dose was not related except to percentage impaired, motivation/organization. Late-life neurocognitive function in atomic bomb survivors exposed as children was associated with age, but not clearly with radiation dose. More studies are needed to evaluate the effect of low-dose radiation during childhood on long-term neurocognitive function.