This study reports 3-year effects of a parent-teen cognitive/behavioral treatment for adolescent attention-deficit/hyperactivity disorder (ADHD), blended with Motivational Interviewing (Supporting Teens' Autonomy Daily [STAND]), vs Usual Care (UC) in 4 community clinics. This was a randomized clinical trial with double randomization of adolescents and therapists to STAND vs UC. Participants were 278 culturally diverse adolescents diagnosed with DSM-5 ADHD and 82 community therapists. Long-term effects on outcomes and theorized mechanisms were assessed at approximately 3 years post baseline (mean age= 16.94 years, SD= 1.69): ADHD severity (parent-rated), parent-teen conflict (parent/adolescent-rated), organization, time management, and planning skills (OTP; parent-rated), treatment and school enrollment (parent/adolescent-reported), and ADHD diagnostic persistence (clinician-determined). Therapist licensure was examined as a treatment moderator. Intent-to-treat (ITT) and per protocol analyses (n = 225; participants initiating treatment after agency intake) were conducted. As in the original trial, ITT analyses indicated no long-term group by time effects. However, STAND (vs UC) led to superior long-term outcomes when therapists were licensed (22% of sample) vs unlicensed for parent-rated hyperactivity/impulsivity (d= 0.39; adolescent-rated parent-teen conflict: d= 0.27, and parent-rated OTP skills: d= 0.79). Previously reported post-treatment group differences on medication engagement were non-significant at the 3-year follow-up. Although STAND did not outperform UC overall, group by licensure interactions indicate specific long-term impacts on ADHD symptoms, executive function skills such as OTP, and parent-teen conflict, extending this trial's acute effects and replicating previous findings. Clinicians in community settings might recommend adjunctive cognitive/behavioral treatment to adolescents with ADHD to maximize long-term outcomes. However, additional efforts are needed to facilitate effective implementation by unlicensed clinicians. STAND Community Trial (STAND); https://clinicaltrials.gov; NCT02694939. Long-term Follow-up of MI-based Behavioral Intervention Delivered in Community Mental Health; https://osf.io; h5w6f. We worked to ensure sex and gender balance in the recruitment of human participants. We worked to ensure race, ethnic, and/or other types of diversity in the recruitment of human participants. We worked to ensure that the study questionnaires were prepared in an inclusive way. One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented racial and/or ethnic groups in science. One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented sexual and/or gender groups in science. One or more of the authors of this paper self-identifies as living with a disability. We actively worked to promote sex and gender balance in our author group. We actively worked to promote inclusion of historically underrepresented racial and/or ethnic groups in science in our author group. While citing references scientifically relevant for this work, we also actively worked to promote sex and gender balance in our reference list. The author list of this paper includes contributors from the location and/or community where the research was conducted who participated in the data collection, design, analysis, and/or interpretation of the work.