Previous studies have linked higher generativity with better mental well-being. However, most of these studies investigated the predictive role of generativity in well-being, while the converse relation, that is, how mental well-being contributes to generativity, has been ignored. This study first investigated the structure and stability of multidimensional mental well-being, that is, emotional (including happiness, life satisfaction, and positive and negative mood), psychological, and social well-being and the absence of depressive feelings, from age 42 to 61. Second, longitudinal associations between mental well-being and generativity were examined. The data (n = 301) utilized in this study were drawn from the Jyväskylä Longitudinal Study of Personality and Social Development and were collected using self-report questionnaires and psychological interviews at the ages of 42, 50, and 61. Using structural equation modeling, the multidimensional structure of mental well-being showed partial strong factorial invariance and high stability from age 42 to 61. The associations between mental well-being (both the multidimensional factor and the single indicators) and generativity were tested using the random intercept cross-lagged panel model. Stable, trait-like associations were found between multidimensional, emotional, and psychological well-being and generativity. The longitudinal results showed that social well-being at age 42 predicted generativity at age 50. To conclude, multidimensional mental well-being seemed to remain stable from middle adulthood to the beginning of late adulthood. Furthermore, mental well-being and generativity may be linked at both the between- and within-person levels. In particular, social well-being appeared to be a resource through which individuals could increase their generativity.