Older adults may be confronted with a variety of negative aging stereotypes (e.g., “forgetful,” “physically frail,” and “lonely”) almost every day. While experimental studies have demonstrated the impact of negative aging stereotypes on older adults’ cognitive performance, the relationship between multiple negative aging stereotype experiences and cognitive functioning in older people’s daily lives is largely unknown. Using a 1-week daily diary study approach, the present studies examined the association between experiencing daily negative aging stereotypes and real-life cognitive functioning and the mediating role of daily negative affect. We also examined whether these associations differed in response to negative aging stereotype diversity, which refers to the breadth and evenness of negative aging stereotypes that older adults experience in 1 week. In Study 1 (n = 50), we explored the negative aging stereotypes that older adults often experience in their daily lives. These stereotypes can be seen as a measure of daily stereotype experiences. Then in Study 2, 100 older adults reported their everyday experiences of negative aging stereotypes, positive or negative affect, and cognitive failures for 1 week. The results showed that the more negative aging stereotype experiences older adults had in 1 day, the more cognitive failures occurred that day and that daily negative affect mediated this association. Further, the direct and indirect relationships between daily negative aging stereotype experiences and daily cognitive failures were significant in older adults with high negative aging stereotype diversity but not in those with low negative aging stereotype diversity. This study provides new insights for studies on aging stereotypes threat and offers theoretical guidance for future interventions for cognitive health in older adults and, in turn, contributes to promoting healthy aging in older adults.