Objectives: Mastery beliefs are known to contribute to healthy aging. However, it is an open question whether individual mastery-health associations impact the health of close long-term partners. Method: We applied actor-partner interdependence models to 4-wave, 6-year longitudinal dyadic data from married and cohabitating partners in the Health and Retirement Study (N = 1,981 partners; age at baseline: M = 67 years, SD = 8.93, range 50-94 years). Results: Higher individual mastery beliefs were associated with better individual physical health and health behaviors. Higher mastery beliefs were associated with subsequent increases in light physical activity. Having a partner with higher levels of mastery was uniquely associated with fewer functional limitations, better self-rated health, and more physical activity. Actor x Partner interaction effects for functional limitations indicated multiplicative associations of actor and partner mastery with health. Of note, mastery-health associations for individuals and their partners were invariant across age, gender, education, employment status, perceived stress over ones own and partners health, and cognition. Discussion: Findings suggest that partner mastery beliefs matter for the health (behaviors) of older adults. We discuss possible mechanisms underlying partner interrelations in mastery and health, their age invariance, and consider implications arising from our results.