Severe methylmercury poisoning occurred in Minamata and neighboring communities in Japan during the 1950s and 1960s. A considerable number of children were born with conditions resembling cerebral palsy, later known as congenital Minamata disease. Although surviving patients are now in their 50s or 60s, few assessments of functional capacities in daily living have been performed. We assessed the changes in Activities of Daily Living (ADL) status of 11 patients over a 10-year period. For assessment of ADL, we applied the Barthel Index (BI) and the Functional Independence Measure (FIM). We obtained the patients' current and previous status information by interview of their caregivers or from medical records and then compared them using the Wilcoxon signed-rank test. Both ADL measures, including the status related with cognition, had significantly declined during the 10 years. The same was true for the overall BI score (p = 0.01). Similarly, all functions of the FIM scores declined (i.e., self-care, sphincter control, mobility, locomotion, communication, and social cognition), and for FIM physical and cognition subscores as well as FIM total score, the declines were statistically significant. The present study indicates that the ADL status of congenital Minamata disease patients, now in their 50s or 60s, has substantially declined during the last 10 years; a decline that was much steeper in comparison with expectation in subjects of similar ages, but in agreement with accelerated ageing also reported in subjects with cerebral palsy, past polio infection, or epilepsy. While already incapacitated due to the prenatal methylmercury poisoning, their accelerated ageing may suggest that patients with developmental neurotoxicity have less reserve capacity to compensate for normal ageing. These patients will need continuous and increasing medical and welfare support in the community.