We aimed to estimate the impact of social isolation on cognitive function and mental health among older adults during the two-year-and-a-half COVID-19 period. Pubmed Central, Medline, CINAHL Plus and PsychINFO were searched between March 1, 2020, and September 30, 2022. We included all studies that assessed proportions of older adults with the mean or the median with a minimum age above 60 reporting worsening cognitive function and mental health. Thirty-two studies from 18 countries met the eligibility criteria for meta-analyses. We found that the proportions of older adults with dementia who experienced worsening cognitive impairment and exacerbation or new onset of behavioral and psychological symptoms of dementia (BPSD) were approximately twice larger than that of older adults with HC experiencing SCD and worsening mental health. Stage of dementia, care options, and severity of mobility restriction measures did not yield significant differences in the number of older adults with dementia reporting worsening cognitive impairment and BPSD, while the length of isolation did for BPSD but not cognitive impairment. Our study highlights the impact of social isolation on cognitive function and mental health among older adults. Public health strategies should prioritize efforts to promote healthy lifestyles and proactive assessments.