Objective: Drinking goals set at treatment onset predict treatment outcome in patients with alcohol use disorders. Yet the cognitive constructs of goal setting and goal attainment are understudied in young adult drinkers. This study sought to examine how the interplay of goal setting and goal attainment during treatment impacts treatment outcome in a sample of young adult heavy drinkers. Method: Participants were 128 young adult heavy drinkers (Mage = 21.5 years) who participated in a double-blind, placebo-controlled, 8-week efficacy trial of naltrexone plus brief counseling. Participants were not required to be interested in changing their drinking for inclusion. Drinking goals were assessed at baseline, midtreatment, and end of treatment. Outcomes were peak drinking, typical drinking, and drinking frequency. Results: Results from PROCESS serial, multiple mediator models showed that midtreatment goal setting and goal attainment collectively predicted peak drinking (b = 0.87, 95% CI [0.40, 1.37]) and drinking frequency (b = 0.66, 95% CI [0.37, 1.06]). Only midtreatment goal setting mediated the relationship between baseline goal setting and typical drinking (b = 0.35, 95% CI [0.10, 0.85]). Participants who set more ambitious drinking goals at baseline were more likely to set subsequent, ambitious goals; more ambitious goals at midtreatment were associated with better treatment outcomes. Conclusion: Setting initial, ambitious goals led to further ambitious goals, which ultimately contributed to lower levels of drinking. Thus, cognitive processes during treatment may be an important target of intervention efforts. For example, the inclusion of goal-setting exercises during treatment could serve to improve intervention effects.