Background: ADHD is a generally persistent neurodevelopmental condition that can have significant consequences if left untreated, and medication, often in conjunction with psychotherapy, is the first-line treatment with well-established efficacy. Adolescents and young adults (AYAs) represent a particularly vulnerable and understudied subgroup and there remains a significant concern regarding transition to treatment independence, treatment engagement and medication adherence in this population. We report the results from a pilot trial evaluating Customized Adherence Enhancement-ADHD (CAE-ADHD), a psychosocial intervention for AYAs with ADHD, aimed to increase independence in managing ADHD and improve ADHD symptoms. Methods: We used an iterative process to modify an existing psychosocial intervention to address engagement, independence, and adherence in AYAs with ADHD, originally developed for AYAs with bipolar disorder. To develop the CAE-ADHD intervention, we conducted focus groups and cognitive interviews with AYA with ADHD. We then conducted a pilot feasibility and efficacy study in AYAs with ADHD (n = 12) with follow-up visits at 8 and 12 weeks into the study. Results: The sample consisted of 12 participants ages 16 to 24 years with a diagnosis of ADHD, obtained via self-report, and currently prescribed stimulant medication. Mean age of the sample was 19.5 years, 58.33% female, 25% non-White. Participation and retention were strong with 93.33% of sessions completed overall and 10/12 participants completing all sessions. AYAs showed a trend toward improvement in ADHD symptoms as measured by the ADHD RS-5, statistically significant improvement in transition readiness and ADHD knowledge. There was no significant change in adherence. Conclusions: This first of its kind feasibility and efficacy pilot study for a novel intervention, CAE-ADHD, found high engagement and improvement in ADHD symptoms, transition readiness, and ADHD knowledge from baseline to week 12.