Previous research suggests that routine psychosocial care for adolescents with attention-deficit/hyperactivity disorder (ADHD) is an eclectic and individualized mix of diluted evidence-based practices (EBPs) and low-value approaches. This study evaluated the extent to which a community-delivered EBP and usual care (UC) for adolescents with ADHD produce differential changes in theorized behavioral, psychological, and cognitive mechanisms of ADHD. A randomized community-based trial was conducted with double randomization of adolescent and community therapists to EBP delivery supports (Supporting Teens’ Autonomy Daily [STAND]) versus UC delivery. Participants were 278 culturally diverse adolescents (ages 11–17) with ADHD and caregivers. Mechanistic outcomes were measured at baseline, post-treatment, and follow-up using parent-rated, observational, and task-based measures. Results using linear mixed models indicated that UC demonstrated superior effects on parent-rated and task-based executive functioning relative to STAND. However, STAND demonstrated superior effects on adolescent motivation and reducing parental intrusiveness relative to UC when it was delivered by licensed therapists. Mechanisms of community-delivered STAND and UC appear to differ. UC potency may occur through improved executive functioning, whereas STAND potency may occur through improved teen motivation and reducing low-value parenting practices. However, when delivered by unlicensed, community-based therapists, STAND did not enact proposed mechanisms. Future adaptations of community-delivered EBPs for ADHD should increase supports for unlicensed therapists, who comprise the majority of the community mental health workforce.