Objective: The purpose of this study was to investigate the effects of community-based cognitive rehabilitation therapy based on occupational therapy applied to cognitive function, physical function, and occupational performance in elderly people with mild cognitive impairment and dementia. Methods: We conducted group cognitive rehabilitation therapy in 10 public health centers in Gangwon-do, over a period of approximately 9 months, from March 1 to November 31, 2016. The intervention was provided to 95 subjects in total, one session per week, 120 min per session, for an average of 20.8 sessions. We compared the cognitive function, physical flexibility, balance ability, and occupational performance of the subjects. Results: In the testing of cognitive function, the MMSE-DS attention scores improved significantly. In terms of physical function, there was a significant shortening of the measured distance in the wall test, which measures the flexibility. In the balance testing, the measured time during a one-legged standing test improved significantly, and the measured time during a 10-foot timed walk test was significantly shortened. In the occupational performance testing, COPM occupational performance scores improved significantly. Conclusion: Occupational therapy-based group cognitive rehabilitation therapy was shown to be effective at maintaining and improving the attention, physical flexibility, balance ability, and occupational performance in elderly people with mild cognitive impairment and dementia, suggesting that this treatment method can be utilized as a community-based clinical model.