This study examined the relative contribution of two dimensions of parent engagement, attendance and homework adherence, to parent and child treatment response and explored whether early engagement was a stronger predictor of outcomes than later engagement. The sample consisted of parents of participants (n = 92; M age = 9.4 years, SD = 1.27; 67% male, 69% White) in a 12-session evidence-based family-school intervention for children with attention-deficit/hyperactivity disorder. Attendance was assessed using clinician records, and homework adherence was measured by rating permanent products. Outcomes included parent and teacher ratings of family involvement in education, parenting practices, and child functioning. Accounting for the contributions of baseline scores and attendance, homework adherence was a significant predictor of parental self-efficacy, the parent-teacher relationship, parenting through positive involvement, and the child's inattention to homework and homework productivity. Accounting for the contribution of baseline scores and homework adherence, attendance was a significant predictor of one outcome, the child's academic productivity. Early homework adherence appeared to be more predictive of outcomes than later adherence, whereas attendance did not predict outcomes during either half of treatment. These results indicate that, even in the context of evidence-based practice, it is the extent to which parents actively engage with treatment, rather than the number of sessions they attend, that is most important in predicting intervention response. Because attendance is limited as an index of engagement and a predictor of outcomes, increased efforts to develop interventions to promote parent adherence to behavioral interventions for children are warranted.