OBJECTIVE Successful self-management of type 1 diabetes requires cognitive skills such as executive functioning (EF). In the transition to adolescence, youth take over responsibility for diabetes management. We set out to test: 1) the association between EF and glycemic control over time and 2) whether this association was moderated by: a) youth, shared, or parent responsibility for diabetes management and b) youth’s age. RESEARCH DESIGN AND METHODS Within the Diabetes IN DevelOpment study (DINO), parents of youth with type 1 diabetes (8–15 years at baseline; N = 174) completed a yearly assessment over 4 years. Glycemic control (HbA1c) was derived from hospital charts. Youth’s EF was measured using the Behavior Rating Inventory of Executive Functioning (BRIEF)-parent report. The Diabetes Family Responsibility Questionnaire (DFRQ)-parent report was used to assess diabetes responsibility (youth, shared, and parent). Linear generalized estimating equations were used to analyze data including youth’s sex, age, and age of diabetes onset as covariates. RESULTS Relatively more EF problems are significantly associated with higher HbA1c over time (β = 0.190; P = 0.002). More EF problems in combination with less youth responsibility (β = 0.501; P = 0.048) or more parental responsibility (β = −0.767; P = 0.006) are significantly associated with better glycemic control over time. Only age significantly moderates the relationship among EF problems, shared responsibility, and glycemic control (β = −0.024; P = 0.019). CONCLUSIONS Poorer EF is associated with worse glycemic control over time, and this association is moderated by responsibility for diabetes management tasks. This points to the importance of EF when youth take over responsibility for diabetes management in order to achieve glycemic control.