Background: The purpose of this study was to investigate cognitive emotion regulation in adolescents with chronic illness and their parents. Methods: Eighty-five young people (mean = 15.86 years, standard deviation = ± 1.42, girls 65.88%) with chronic illnesses (inflammatory bowel disease n = 40 or type 1 diabetes n = 45), and their parents (mean = 46.06 years, 87.06% mother) completed the Cognitive Emotion Regulation Questionnaire (CERQ) for themselves and the Inventory of Quality of Life in Children and Adolescents (ILC) questionnaire adolescent and parent version. We conducted two hierarchical linear regression analyses with “enter” method. The CERQ scales and the diagnosis of chronic disease were chosen as independent variables, and the total ILC score in the first analysis and the ILC proxy score in the second analysis were chosen as dependent variables. Results: Among adolescents, cognitive emotion regulation strategies such as self-blame, positive reappraisal, and catastrophizing have been proven to be predictors of their own quality of life; however, parental self-blame was also found to be a predictor of adolescents’ quality of life. Parental rumination and positive refocusing have been shown to be predictors of how parents rate their child’s quality of life. Conclusions: The present study sheds light on cognitive emotion regulation strategies in adolescents with chronic illness and their parents that have a significant impact on the development of young people’s quality of life.