We investigated whether parenting and child behavior improve following psychosocial treatment for Attention-Deficit/Hyperactivity Disorder, Predominantly Inattentive Presentation (ADHD-I) and whether parenting improvements mediate child outcomes. We analyzed data from a randomized clinical trial investigating the efficacy of a multicomponent psychosocial intervention (Child Life and Attention Skills, CLAS, n = 74) in comparison to Parent-Focused Treatment (PFT, n = 74) and treatment as usual (TAU, n = 51) for youth with ADHD-I (average child age = 8.6 years, range 7–11 years, 58 % boys). Child and parent/family functioning were assessed prior to treatment, immediately following treatment, and at follow-up into the subsequent school year using parent and teacher reports of inattention, organization, social skills, academic competency (teachers only), parenting daily hassles, and positive and negative parenting behaviors (parents only). Both treatment groups improved on negative parenting and home impairment, but only CLAS families also improved on positive parenting as well as academic impairment. Improvements in positive and negative parenting mediated treatment effects on child impairment independent of improvements in child inattention, implicating parenting as an important mechanism of change in psychosocial treatment for ADHD-I. Further, whereas parent-focused training produces improvements in negative parenting and impairment at home for children with ADHD-I, a multicomponent approach (incorporating child skills training and teacher consultation) more consistently produces improvements at school and in positive parenting, which may contribute to improvements in social skills into the next school year.