Aims: This study addressed the age of onset of conduct disorder (CD) and oppositional defiant disorder (ODD) in treatment-seeking substance use disorder (SUD) patients with and without adult attention-deficit/hyperactivity disorder (ADHD) and its association with early onset of SUD. Methods: We examined data from the 2nd International ADHD in Substance Use Disorders Prevalence Study, including 400 adults in SUD treatment from Puerto Rico, Hungary, and Australia. ADHD, SUD, and CD/ODD were assessed with the Conners Adult ADHD Diagnostic Interview for DSM-IV, the MINI International Neuropsychiatric Interview, and the K-SADS, respectively. Cox regression analyses modeled time to emergence of CD/ODD separately for SUD patients with and without adult ADHD. Linear regression models examined associations between age of onset of SUD and presence of ADHD and adjusted for sex, age, and country. To assess the mediating role of CD/ODD on the association of ADHD with onset of SUD, adjusted regression models were estimated. Results: Treatment-seeking SUD patients with ADHD presented an earlier onset of CD/ODD compared with those without ADHD. CD/ODD symptom loads were higher among the SUD and ADHD group. Age of first substance use and SUD were significantly earlier in SUD patients with ADHD, and these findings remained significant after adjustment for demographics and coexisting CD/ODD. Conclusions: ADHD is associated with earlier onset of SUD as well as with an earlier onset of more frequent and more severe disruptive behavioral disorders. These findings may inform preventive interventions to mitigate adverse consequences of ADHD.