Background <p>Atomic bomb survivors (ABSs) in Hiroshima are facing climate change-related natural disasters and emerging infectious diseases. The cognitive function of aging ABSs is vulnerable to the inevitable environmental changes caused by the 2018 Japan floods and COVID-19 pandemic.</p> Aims <p>This study examined the effects of these two disastrous events on cognitive function.</p> Methods <p>This retrospective cohort study included all verified individuals utilizing Long-Term Care Insurance services in Hiroshima Prefecture. The observation period was from January 2018 to December 2022. The participants were divided into three groups: ABSs, welfare recipients (WRs), and others. The objective variable was moderate or severe cognitive deterioration. We performed a difference-in-differences analysis using logistic regression models to investigate the effect of the two disastrous events on cognitive function compared with the effect of the other groups.</p> Results <p>There were 184,252 participants, including 16,306 ABSs (8.8%) and 8,509 WRs (4.6%). The difference-in-differences analysis showed no statistically significant effect of the 2018 Japan floods. The analysis also revealed that moderate cognitive decline among ABSs and WRs decreased during the COVID-19 pandemic (2020, 2021, and 2022). Moreover, severe cognitive decline among ABSs decreased after the onset of the COVID-19 pandemic (2020 and 2021).</p> Discussion <p>Although many older adults experienced cognitive exacerbations during the COVID-19 pandemic, ABSs had a lower risk of cognitive decline than those of non-WRs. However, no significant changes were observed during the 2018 Japan floods.</p> Conclusions <p>ABSs had a reduced risk of cognitive decline during the pandemic compared with that of the other groups.</p>