Do adolescents with histories of attention-deficit/hyperactivity disorder (ADHD) who subsequently enrol in post-secondary education differ from their peers with ADHD histories who do not enrol? This thesis explored ADHD histories and post-secondary enrolment as factors that may shape the developmental trajectories of academic outcomes and mental health. Using longitudinal data from the Multimodal Treatment Study of Children with ADHD (MTA) and latent curve modelling, I explored ADHD symptoms, academic outcomes, and anxiety and depression symptoms from ages 9 through 17 for four groups: adolescents with ADHD histories and eventual post-secondary enrolment (n = 332), adolescents with ADHD histories but without post-secondary enrolment (n = 188), adolescents without ADHD histories but with eventual post-secondary enrolment (n = 205), and adolescents with neither ADHD histories nor post-secondary enrolment (n = 45). From ages 9 through 17, adolescents with ADHD histories who did not eventually enrol in post-secondary studies reported more ADHD symptoms and worse academic outcomes than those who did eventually enrol. Anxiety and depression symptoms were not significantly different across the four groups. Machine learning analyses to classify adolescents with ADHD histories as either "post-secondary" or "no post-secondary" had parallel results, identifying the most important features as academic achievement and ADHD symptoms. In general, academic outcomes were better for adolescents who eventually enrolled in post-secondary studies, regardless of ADHD histories. Among those with ADHD histories, mathematics and English grades declined for those who did not eventually attend post-secondary studies, whereas high school GPA improved for those who did eventually attend. Thus, to ensure students have an equal opportunity for long-term academic outcomes, it may be particularly important for parents, educators, and clinicians to pay attention to students who have low academic achievement scores, students who have declining grades in middle school, and students with more severe ADHD symptoms. Interventions targeted toward improving academic outcomes, persistent treatment, and coaching strategies may be important ways to support the academic achievements of adolescents with ADHD.