The goal of this study was to examine whether parent attention-deficit/hyperactivity disorder (ADHD) symptoms impact parenting outcomes (i.e., parenting practices and stress) after group-based Parent Child Interaction Therapy (PCIT). The study included parents of preschoolers (Mage = 4.97) with autism spectrum disorder and co-occurring externalizing behavior problems (ASD + EBP; 37) and parents of children with EBP-only (n = 41) who participated in a multimodal behavioral intervention, including an 8-week group-based PCIT. Parents reported on their own ADHD symptoms, practices, and stress at pre-and-post-treatment. Accounting for child EBP, parental ADHD symptoms were predictive of higher negative and lower positive parenting at post-treatment, for parents in the ASD + EBP group but not the EBP-only group. Parental ADHD symptoms also predicted higher parenting stress for the ASD + EBP group only. Results of mediational analyses indicated that for the ASD + EBP group, the indirect effect of parental ADHD symptoms on negative and positive parenting through parenting stress was significant. Findings highlight the differential impact of parental mental health challenges on parenting outcomes. Specifically, parental ADHD symptoms seem to have a larger impact on treatment outcomes for parents of children with co-occurring presentations, which seems to be partially mediated by parenting stress.