Minors entering treatment for alcohol and other drug (AOD) use disorders tend to smoke at high rates, and many have comorbid attention deficit hyperactivity disorder (ADHD). Clear-air laws force patients to refrain from smoking on the premises of AOD treatment facilities, which may hinder the progress of treatment-seeking populations who smoke and struggle with ADHD comorbidity in particular. This study explores clinical characteristics associated with smoking among youths presenting for residential treatment, clinical characteristics associated with smoking cessation, and the impact of smoking cessation with ADHD comorbidity on AOD treatment response. Participants were 195 adolescents (52% female, aged 14–18 years) court-referred to residential treatment. Data were collected at intake, prospectively each week for the 10-week treatment period, and at discharge. Two-thirds (67%) of the enrollment sample entered treatment smoking half a pack a day on average, a large proportion (50%) of which did not smoke during treatment. ADHD patients were more likely to smoke before and during treatment except for those who got active in service and step-work. Quitting smoking did not adversely affect AOD outcomes and was associated with better prognosis of lowered AOD cravings for youths with and without ADHD. Smoking cessation during adolescent AOD treatment is recommended with provision of pharmaceutical and/or behavioral modalities that reduce nicotine withdrawal.