The effectiveness of school-based universal prevention programs is frequently diminished due to low-quality implementation. Organizational factors support high-quality implementation because of their broad influence across implementers. Conceptually, implementation leadership (i.e., behaviors that prioritize, reward, and support evidence-based practice [EBP] implementation) works to embed a favorable implementation climate (i.e., implementers' collective perceptions that their organization prioritizes, rewards, and support EBP implementation) leading to improved implementation citizenship behavior and attitudes toward EBP. This organizational implementation process model has some empirical support but has not been tested in a multilevel framework or related to hypothesized attitudinal and behavioral outcomes. The sample included 319 teachers across 39 US public elementary schools; all were implementing Schoolwide Positive Behavior Interventions and Supports. Multilevel mediation (level 1 = teacher, level 2 = school) was used to test the indirect association of implementation leadership on implementation-related attitudes and behaviors via implementation climate across two time points (fall and spring). At the school level, the organizational implementation process model was validated related to implementation citizenship behavior, but not attitudes toward EBP. At the teacher level, the process model was validated related to both outcomes, and there was a significant direct effect of implementation leadership on attitudes toward EBP. Developing strong leaders for implementation seems key to achieving high-quality EBP implementation. Implications for schools, principal training, and research are discussed.