This study examined functional outcomes for children with ADHD by comorbidity status. Children with ADHD (5–13 years) were recruited from 21 pediatric practices and followed up 12 months later (n = 199). Parent and teacher-reported baseline and 12 month surveys measured peer problems, daily functioning, quality of life (QoL), parent mental health, and family QoL. The Anxiety Disorders Interview Schedule for Children IV assessed mental health comorbidities at baseline. Linear regression models were conducted, adjusting for socio-demographics, ADHD severity, and baseline functioning (where possible). In adjusted analyses, children with ADHD and co-occurring internalizing and externalizing comorbidities had poorer QoL, greater peer problems, and poorer family QoL, compared to children with ADHD alone. The parents of children with ADHD and internalizing and externalizing comorbidities alone, also reported poorer family QoL, compared to children with ADHD alone. Children with ADHD and co-occurring internalizing and externalizing comorbidities appear particularly vulnerable to poorer functioning.