This study examines associations between perceived day-to-day age discrimination, positive well-being, and physical health over a 20-year span. Data came from all three waves of the National Survey of Midlife Development in the United States (1995–2014). Generalized structural equation modeling was used to analyze 6,016 observations of 3,102 participants and test associations between age discrimination and (a) psychological well-being and positive affect, and (b) self-rated health, instrumental activities of daily living, and chronic conditions. Associations were also examined between the well-being measures and all three health outcomes. Between-persons and within-persons effects were modeled separately but simultaneously. Both between-persons and within-persons results revealed numerous significant associations between age discrimination and physical health, although results were stronger between-persons. Moreover, hypothesized associations of age discrimination with well-being, and of well-being with physical health, were supported both between- and within-persons. Findings suggest diminished well-being may be one mechanism whereby age discrimination harms health.