This study examines the long-term associations among parental support, extra-familial partner support, and the trajectories of sense of mastery from adolescence to the early adulthood years. Ten waves of panel data collected over a 16-year period from the Iowa Family Transitions Project (N = 527) were used to test the hypotheses. Results indicated that parental support in adolescence was initially associated with higher levels of sense of mastery and with greater extra-familial partner support during the transition to adulthood. Higher extra-familial partner support was subsequently associated with an increase in sense of mastery during the transition to adulthood. Overall, our results underscore the importance of mastery in smoothing the adjustment to adulthood and the importance of supportive relationships in shaping a healthy sense of mastery. (Contains 1 figure and 2 tables.)