Background: Nowadays, most adolescents with mild congenital heart disease (CHD) undergo medical or surgical correction in early childhood for their congenital anomalies. There is a need to examine the determinants of exercise behaviour among adolescents (CHD) who are able to exercise. Aims: The aims of this study were to examine determinants of exercise among adolescents with mild CHD, including personal beliefs about exercise, interpersonal influences on exercise, and availability of physical environments for exercise, based on social-cognitive theory. Method: A repeated-measures study was conducted at three medical centres in Taiwan. All participants, 126 adolescents with CHD aged 12 to 18 years, had a cardiologist's recommendation of exercise with no limits or limits only on vigorous exercise. Self-administered questionnaires were completed during a summer vacation and again in the fall semester of school along with a 7-day exercise log. The data were analysed using structured equation modelling. Results: The effect of interpersonal influences on total exercise and moderate-to-vigorous exercise was mediated by personal exercise beliefs. Peer influences had significant effects on moderate-to-vigorous exercise through the mediator of perceived exercise self-efficacy. There was no significant difference in the determinants of exercise behaviour during two periods. Personal exercise beliefs, particularly perceived exercise self-efficacy, played the most important role in determining the performance of moderate-to-vigorous exercise. Conclusions: The social-cognitive determinants may serve as a clinical reference for promoting exercise among adolescents with mild CHD.