As the first stage of substantive theory building, this study explored the behavioral responses of people with long-term weight concerns in using mHealth to increase physical activity and how they perceive the role and social interaction of mHealth engagement, grounded in real-life experiences in a New Zealand context. A constructivist grounded theory method was adopted. Fifteen participants with long-term weight concerns and personal experience using mobile health to increase physical activity participated in in-depth interviews. Four themes and nine categories were conceptualized as proposed. Individuals' behavioral responses were principally driven by intrinsic factors, that is, individual concerns about long-term weight issues and self-efficacy derived from perceptions of the impact on wellness. Mobile health acts as an external trigger that engages and impels individuals to adopt behavior change in forming long-term behavioral routines. The role of mobile health in increasing physical activity and improving overall wellness is broadly acknowledged and facilitates, to some extent, the social interactions among whānau and the wider community.