Objectives: Habit has been proposed as an explanation of why prior behavior is a good predictor of later behavior. This study examined whether habit mediates the relationship between prior and later physical activity (PA), within the framework of the attitude-social influences-efficacy (ASE) model and the theory of planned behavior (TPB). Design: A longitudinal design was used. Methods: A total of 1976 older adults completed questionnaires on ASE/TPB constructs and PA at baseline, intention at three months, habit at six months, and PA at twelve months. Results: Path analyses showed that habit significantly mediates the relationship between prior and later PA, after ASE/TPB variables were taken into account. Conclusions: Habit is a partial solution to the question why prior PA is a good predictor of later PA. It is recommended to incorporate habit into the ASE/TPB model.