We investigated treatment effects on parenting self-efficacy and parent cognitive errors, and whether these parent cognitions are related to short- and long-term outcomes in parenting behaviors in psychosocial treatment for youth with Attention-Deficit/Hyperactivity Disorder, Predominantly Inattentive Presentation (ADHD-I). In a randomized controlled trial across 2 sites (University of California, San Francisco and University of California, Berkeley), 199 children between the ages of 7 and 11 were randomized to the Child Life and Attention Skills program (CLAS, n = 74), Parent-Focused Treatment (PFT, n = 74), or Treatment as Usual (TAU, n = 51). Parents reported on self-efficacy, cognitive errors, positive parenting, and negative parenting prior to treatment, immediately after treatment, and in the next school year at follow-up. Compared to TAU, CLAS and PFT had higher post-treatment parenting self-efficacy, and CLAS alone had lower post-treatment parent cognitive errors. At follow-up, only CLAS had improved parent cognitive errors compared to TAU. No other between-group differences were found in parenting self-efficacy or cognitive errors. Improved parenting self-efficacy was associated with improved post-treatment negative parenting outcomes for PFT and CLAS, and improved parent cognitive errors were also related to improvements in positive and negative post-treatment parenting outcomes for CLAS. Post-treatment parenting self-efficacy mediated follow-up negative parenting outcomes for CLAS and post-treatment parent cognitive errors mediated improved follow-up positive and negative parenting outcomes for CLAS. PFT and CLAS led to enhanced parenting self-efficacy, and CLAS appears especially robust in improving parent cognitive errors both in the short- and long-term. Pathways provide support for the possibility of parent cognitions as mediators of treatment effects on parenting; clinical focus on such cognitions may be useful.