This study examines whether a community-based physical activity intervention influenced movement in stages of change in physical activity in an adult population, whether any such effect was mediated by psychosocial influences, and whether any such mediations were moderated by sociodemographic or anthropometric factors. The 3-year-long pseudoexperimental intervention included physical activity, communication, environmental, and participatory components. Questionnaires assessed stages of change in physical activity and potential psychosocial mediators. Regression analyses revealed a significant favorable intervention effect on stages of change, partially mediated by enhanced support from family, perceived control, and physical activity identity. No moderation effects by gender, age, ethnicity, education, or body mass index were observed. Support from family, perceived control, and identity might be important intervention target points for inducing forward movement in the stages of change in physical activity, regardless of sociodemographic and anthropometric background factors.