Background: Irritability is frequently comorbid with ADHD. Although irritability alone has been linked to deleterious mental health and adaptive issues, the joint developmental course of ADHD and irritability symptoms during childhood as well as its association with later mental health and suicidal outcomes is not fully understood. We aimed to describe the developmental trajectories of childhood ADHD and irritability symptoms and to quantify their association with adolescent mental health and suicidal outcomes. Methods: The Quebec Longitudinal Study of Child Development (QLSCD) included 1407 participants from the general population followed up from age 5 months to 17 years. We used a multitrajectory approach to identify developmental trajectories of childhood (6-12 years) ADHD and irritability symptoms. Outcome measures were adolescent (13-17 years) mental health (psychiatric symptoms/functional impairment) and suicidal outcomes. Results: We identified distinct developmental profiles: combined absent or very low ADHD and absent or very low irritability (940 [66.8%]; reference group), moderately high irritability and low ADHD (158 [11.2%]), moderately high ADHD and low irritability (198 [14.1%]), and combined high ADHD and high irritability (111 [7.9%]). Multivariate modeling showed that, compared to children in the reference group, those in the combined high ADHD and high irritability profile showed higher levels of ADHD continuity (d ranges = 0.40-0.50), externalizing (d ranges = 0.25-0.59), internalizing (d ranges = 0.20-0.29), and functional impairments (d ranges = 0.17-0.48) and suicidal behaviors (odds ratio (OR) = 2.12, confidence interval (CI) = 1.47-3.06) in adolescence. Conclusions: The presence of persistently high levels of irritability along with ADHD symptoms during childhood significantly predicts adolescent ADHD continuity, externalizing, internalizing, and suicidal outcomes. Systematic consideration of irritability when assessing and treating ADHD may improve long-term mental health outcomes.