Contemporary conceptualizations of mindfulness refer to paying attention to the present moment with an open and nonjudgmental attitude. Empirical research is increasingly focusing on mindfulness as a dynamic and multifaceted state that can fluctuate across situations and time. The present study aimed to extend existing knowledge by using experience-sampling methodology to examine state-level mindfulness, pleasant/unpleasant event occurrence, and affective well-being in the everyday lives of middle-aged and older adults. A community-based sample of 157 participants aged 53–86 (M = 69.36, SD = 5.80) was recruited in May–June 2020. Participants completed assessments of multidimensional state mindfulness, hassle and uplift occurrence, and positive and negative affect on their smartphones, four times a day over 10 consecutive days. (Total observations: 4,761; M = 30.32, SD = 8.78.) Multilevel models indicated that the state mindfulness-facets present-moment attention and nonjudgmental acceptance were predictive of greater affective well-being in older adulthood. Furthermore, nonjudgmental acceptance appeared to buffer affective reactivity to daily hassles, and importantly, this effect was stronger at older ages. Mindful states did not appear to provide any further boost to uplift-related mood. The findings suggest that occasions characterized by relatively high present-moment attention and nonjudgmental acceptance may contribute to enhanced emotional well-being in later adulthood. Furthermore, adopting a nonevaluative and accepting orientation toward momentary experiences may be a psychological strategy that has particular utility for mitigating emotional reactivity to daily stressors with increasing age. Future research should investigate multidimensional state mindfulness and affective well-being for middle-aged and older adults participating in mindfulness-based interventions.