Research on mHealth apps provides mixed evidence regarding their effectiveness for behavior change, including physical activity. Synthesizing prior perspectives, we test predictors of tracking app and physical activity intentions (Study 1; n = 658) and their links to everyday mobility (Study 2; n = 418; n = 27,617,440 observations). Study 1 showed that individuals have overlapping perceptions of tracking apps and physical activity. Taking a naturalistic mobile sensing approach, Study 2 found that tracking app and physical activity intentions predicted self-reported physical activity – but not logged movement (i.e., walking, cycling, or running). Tracking app use was not related to the level of logged movement in daily life. However, participants who regularly used tracking apps were more likely to view them as impactful, suggesting that daily mHealth app use is related to perceived (vs. observed) behavior change. Together, our studies illuminate how perceptions of mobility and mobile media – and their effects – can become intertwined in users’ minds.