This study tested an integrated social cognitive model of physical activity intention formation, onset, and maintenance among sedentary participants (N = 465) in a church-based, social cognitive, physical activity intervention. Three separate models were tested via structural equation modeling. Each model provided a good fit to the data. The models explained 28%, 19%, and 9% of the variance in intention formation, physical activity onset, and physical activity maintenance, respectively. Consistent with hypotheses, self-efficacy mediated the effect of positive outcome expectancy on behavioral intention, adoption of behavioral strategies mediated the effect of the intervention on physical activity onset, and maintenance of self-efficacy mediated the effect of the intervention on physical activity maintenance. Contrary to hypotheses, change in self-efficacy from baseline to post-assessment and perceived satisfaction with intervention outcomes did not have effects on physical activity onset or maintenance. The findings provide preliminary evidence that physical activity intention formation, onset, and maintenance are distinct processes driven by different determinants before, during, and following a social cognitive physical activity intervention.