Problematic rates of alcohol, e-cigarette, and other drug use among US adolescents highlight the need for effective implementation of evidence-based programs (EBPs), yet schools and community organizations have great difficulty implementing and sustaining EBPs. Although a growing number of studies show that implementation support interventions can improve EBP implementation, the literature on how to improve sustainability through implementation support is limited. This randomized controlled trial advances the literature by testing the effects of one such implementation intervention—Getting To Outcomes (GTO)—on sustainability of CHOICE, an after-school EBP for preventing substance use among middle-school students. CHOICE implementation was tracked for 2 years after GTO support ended across 29 Boys and Girls Club sites in the greater Los Angeles area. Predictors of sustainability were identified for a set of key tasks targeted by the GTO approach (e.g., goal setting, evaluation, collectively called "GTO performance") and for CHOICE fidelity using a series of path models. One year after GTO support ended, we found no differences between GTO and control sites on CHOICE fidelity. GTO performance was also similar between groups; however, GTO sites were superior in conducting evaluation. Better GTO performance predicted better CHOICE fidelity. Two years after GTO support ended, GTO sites were significantly more likely to sustain CHOICE implementation when compared with control sites. This study suggests that using an implementation support intervention like GTO can help low-resource settings continue to sustain their EBP implementation to help them get the most out of their investment.