BackgroundLeaders can improve implementation outcomes by developing an organizational climate conducive to the implementation of evidence-based practices (EBP). This study tested the lagged associations between individual-level perceptions of implementation leadership, implementation climate, and three anticipated implementation outcomes, that is EBP acceptability, appropriateness, and feasibility.MethodsScreening tools and treatment methods for posttraumatic stress disorder were implemented in 43 Norwegian mental health services. A sample of 494 child and adult mental health care professionals (M= 43 years, 78% female) completed surveys addressing perceptions of first-level leaders’ (n= 47) implementation leadership and their clinics’ implementation climate. Single-level structural equation models estimating both direct, indirect, and total effects were used to investigate whether perceived implementation climate mediated the association between perceived implementation leadership and perceived acceptability, appropriateness, and feasibility of screening tools and treatment methods.ResultsRegarding the treatment methods, implementation leadership was associated with therapists’ perceptions of acceptability, appropriateness, and feasibility. Implementation climate also mediated between implementation leadership and the outcomes. Regarding the screening tools, implementation leadership was not associated with the outcomes. However, implementation climate mediated between implementation leadership and therapists’ perceptions of acceptability and feasibility, but not appropriateness. Analyses with the implementation climate subscales showed stronger associations for therapists’ perceptions of the treatment methods than of screening tools.ConclusionsLeaders may promote positive implementation outcomes, both directly and through implementation climate. With regard to the effect sizes and explained variance, results indicated that both implementation leadership and implementation climate were more strongly associated with the therapists’ perceptions of the treatment methods, implemented by one group of therapists, than the screening tools, implemented by all therapists. This may imply that implementation leadership and climate may have stronger effects for smaller implementation teams within a larger system than for system-wide implementations or when the clinical interventions being implemented are more complex rather than simple ones.Trial registrationClinicalTrials NCT03719651, 25 October 2018.