Objective: Dispositional, intra-personal constructs such as personality traits and generalized beliefs are consistently related to health behaviors, but relatively few studies have tested the theory-based mechanisms by which these constructs relate to health behaviors and compared them across behavior type. In the current study we tested an integrated theoretical model in which belief-based social cognition constructs (attitudes, subjective norms, perceived behavioral control) were proposed to mediate effects of personality traits (conscientiousness, extroversion) and socio-political beliefs (political beliefs, locus of control, free will beliefs) on participation in three health-related behaviors: physical activity, COVID-19 vaccination, and sugar-sweetened beverage restriction. Methods: Proposed integrated model effects were tested in a five-week prospective correlational study. Finnish residents completed measures of personality, socio-political, and social cognition constructs with respect to physical activity participation (N = 557), COVID-19 vaccination uptake (N = 1,115), and sugar-sweetened beverage restriction (N = 558) and self-reported their behavior at follow-up. Results: Structural equation models revealed direct effects of intention on behavior, and of social-cognition constructs on intention, across all behaviors. We also found indirect effects of political beliefs on behavior mediated by social cognition constructs and intentions for COVID-19 vaccination and sugar-sweetened beverage restriction behaviors, indirect effects of conscientiousness on behavior mediated by social cognition constructs and intentions for physical activity and sugar-sweetened beverage restriction behaviors, and indirect effects of health locus of control on behavior mediated by social cognition constructs and intentions for physical activity behavior. Finally, we found a negative total effect of populist beliefs on behavior for COVID-19 vaccination behavior. Conclusion: Findings contribute to an evidence base for the effects of dispositional and social cognition constructs of health behaviors, point to a possible mechanism by which these generalized constructs relate to health behavior, and outline how the pattern of effects varies across the different behaviors.