Physical activity is considered a valuable tool for enhancing life satisfaction. However, the processes linking these constructs likely differ across the adult life span. In older adults the association between physical activity and life satisfaction appears to involve usual levels of physical activity (i.e., a between-person association driven by differences between more and less active people). In younger adults the association has consistently been based on day-to-day physical activity (i.e., a within-person association driven by differences between more and less active days). To resolve this inconsistency, a daily diary study was conducted with a life span sample of community-dwelling adults (age 18-89 years; N = 150) over three 21-day measurement bursts. Usual physical activity was positively associated with life satisfaction in middle and older adulthood; however, this association was not present in young adulthood. When present, this between-person association was mediated by physical and mental health. A within-person association between physical activity and life satisfaction was also present (and did not differ across age). Generally, on days when people were more physically active then was typical for them, they experienced greater life satisfaction. Age differences in life satisfaction followed a cubic trajectory: lower during emerging adulthood, higher during midlife, and lower during older adulthood. This study adds to accumulating evidence that daily fluctuations in physical activity have important implications for well-being regardless of age, and clarifies developmental differences in life satisfaction dynamics that can inform strategies for enhancing life satisfaction.