Peer- and family-based group therapies have been used as separate interventions to improve adjustment and self-management among youth with Type 1 diabetes mellitus. This study replicates a treatment protocol that combined these two types of diabetes management groups, while also using a wait-list control design methodology within an outpatient mental health clinic setting. General psychosocial and diabetes-related variables were assessed at baseline, immediately posttreatment, and 4 months posttreatment. Youths’ medical information, including metabolic control values, was extracted from medical charts for the 6 months prior to baseline and 6 months after treatment ended. At 4 months posttreatment, parents and youth reported increased parent responsibility, and parents reported improved youth diabetes-specific quality of life. Although there were no statistically significant changes in hemoglobin A1c values and health care utilization frequency from 6 months prior to and 6 months posttreatment, other psychosocial changes (i.e., increases in parent responsibility and diabetes-specific quality of life) were documented. Therefore, this treatment was found to be a promising intervention for use in an outpatient clinical setting to aid in improving the psychosocial functioning of youth with Type 1 diabetes mellitus.