• Cognitive and behavioral mechanisms were strongly associated with treatment outcomes. • Participants made large improvements in ADHD knowledge and behavioral strategies. • Additional emphasis on adaptive thinking is needed for students with comorbidities. The purpose of this study was to evaluate trajectories of response for the three theorized mechanisms of clinical change (knowledge, behavioral strategies, and adaptive thinking) associated with the Accessing Campus Connections and Empowering Student Success (ACCESS) intervention for college students with attention-deficit/hyperactivity disorder (ADHD) and their association with treatment outcomes. Participants included 250 college students comprehensively diagnosed with ADHD randomly assigned to ACCESS or to a delayed-treatment control who completed ratings at baseline, end of active treatment, and end of the maintenance phase of treatment (after two semesters). Growth mixture models (GMMs) were used to evaluate trajectories. Participants in ACCESS made significant gains in the use of behavioral strategies and trajectories were associated with large effect size improvements in measures of symptoms and functioning. Participants also made improvements in ADHD knowledge. However, only the knowledge trajectory with rapid improvement displayed significantly better outcomes. Only one trajectory group showed improvement in adaptive thinking with most ACCESS participants remaining stable across time. However, adaptive thinking trajectories were strongly related to both symptom and functional outcomes. ACCESS is associated with large gains in two of the three theorized clinical mechanisms of change, behavioral strategies and ADHD knowledge. Rapid improvement in behavioral strategies was associated with robust improvement in symptoms and functioning. Although improvements in the third mechanism, adaptive thinking, were small, they were strongly associated with outcomes demonstrating the importance of a cognitive-behavioral approach in treating college students with ADHD. [ABSTRACT FROM AUTHOR]