Identity formation is a core developmental task in adolescence and functions as a key resource for transitioning to adulthood. This study investigated how adolescents with congenital cardiac disease form their identity and how it relates to demographic and medical parameters, quality of life, perceived health, depressive symptoms, and loneliness. A total of 429 adolescents aged 14–18 years with congenital cardiac disease and 403 matched controls completed questionnaires on identity and all outcome variables. There were five meaningful identity statuses, similar to those obtained in the control sample, which were found in the patient sample. Of them, two statuses – achievement and foreclosure – were characterised by a strong sense of identity; one status – diffused diffusion – especially was characterised by a weak sense of identity combined with high scores on worry about the future. These identity statuses were differentially related to outcome variables, with individuals in diffused diffusion especially scoring highest on depressive symptoms, problems in school, treatment anxiety, and communication problems with clinicians, and lowest on quality of life. Having a strong sense of personal identity was found to protect against such maladaptive outcomes. In sum, most adolescents with congenital cardiac disease moved through their identity formation process in a similar manner to other adolescents. Adolescents with a diffused identity were particularly at risk of experiencing maladjustment and problems in treatment adherence. Hence, developing intervention strategies to provide continuity of care on the road to adulthood involves paying attention to core developmental tasks, such as identity formation in adolescents with congenital cardiac disease.