Objective: Children and adolescents with a chronic illness (CI) tend to demonstrate diminished physical and social functioning, which contribute to school attendance issues. We investigated the role of social and physical functioning in reducing school absenteeism in children participating in Mastering Each New Direction (MEND), a family-based psychosocial intervention for youths with CI. Methods: Forty-eight children and adolescents with a CI (70.8% female, M[subscript age] = 14.922, SD = 2.143) and their parent(s) completed a health-related quality of life (HRQOL) measure pre- and postintervention. Using multiple mediation, we examined whether parent- and child-rated physical and social HRQOL mediated the relationship between school attendance before and after MEND. Once the mediational model was not supported, we investigated whether HRQOL moderated the relationship between missed school days pre- and postintervention. Results: Neither physical nor social functioning mediated or moderated the relationship between missed school days pre- and postintervention. Instead, higher parent-rated physical functioning directly predicted decreased number of missed school days, while lower parent-rated social and child-rated physical functioning predicted increased missed school days. Conclusions: Parent-perceived HRQOL may have a direct effect on health-related behaviors such as school attendance. Future research should determine whether gains in parent-rated QOL are maintained in the long term and whether these continue to impact markers of functional well-being.