ABSTRACT Background The Summer Treatment Program (STP) is a well-established and efficacious evidence-based treatment for children with attention-deficit/hyperactivity disorder (ADHD) and other behavioral concerns, where counselors and teachers implement a range of evidence-based practices (EBPs) across recreational and educational contexts. Objective The present study evaluates STP staff experiences with training, supervision, and their role in implementing the STP intervention, as well as how their experiences impact their future educational and career goals. Method Participants included 50 STP-Prekindergarten (STP-PreK) staff members (72.0% counselors, 14% lead counselors, 14% teachers) employed at two different sites over two summers. A subset of 14 STP-PreK staff members completed semi-structured follow-up interviews discussing training, supervision, job experience, and their future goals. Results Overall, with the training and supervision received, staff viewed the STP intervention elements as acceptable and easy to implement. Key factors staff highlighted in their learning process included the importance of a supportive environment, opportunities for hands-on learning with feedback, personal self-efficacy, and the role of supportive peers. Staff indicated working at the STP impacted both their use of EBPs and views on their career goals. Conclusions Recommendations for improving STP staff training, supervision, and experiences, as well as implications for future research on the training and supervision of the mental health workforce implementing EBPs with children with externalizing behavioral concerns, are discussed.