This study evaluates transition readiness, medical condition knowledge, self-efficacy, and illness uncertainty in young adolescents (ages 12 to 15 years) with congenital heart disease (CHD), and medical, patient, and parental factors associated with transition readiness. We enrolled 82 patients with moderate or complex CHD (n = 36, 44% male; mean age 13.6 ± 1.3 years), and their parents. Patients completed standardized self-report measures: Transition Readiness Assessment Questionnaire (TRAQ), MyHeart scale, General Self-Efficacy scale, and Children’s Uncertainty in Illness Scale. Parents completed the MyHeart scale and demographic information. Many young adolescents had not discussed transfer with a health care provider (n = 20, 24%) or parent (n = 34, 41%). Transition readiness was higher among patients who were older, more knowledgeable about their condition, had a history of primary cardiac repair and greater self-efficacy, and was lower for boys and patients on cardiac medications. Transition readiness was unrelated to CHD diagnosis and patients’ illness uncertainty. Patients’ self-advocacy skills were superior to their chronic disease self-management skills. Increased parental medical condition knowledge was positively correlated with patient knowledge, and patient–parent discussion of transfer was associated with increased patient’s self-management skills. Transition is not uniformly discussed with young adolescent CHD patients. Parental involvement is correlated with increased transition readiness and patient disease self-management skills. Young adolescent transition programs should focus on education around improving patient medical condition knowledge, promote chronic disease self-management skills development, and include parental involvement.