Although there are a growing number of implementation strategies to increase the adoption, fidelity, and sustainment of evidence-based practices (EBP), they are often complex and bulky, which can interfere with their widespread application. To address these challenges, the Helping Educational Leaders Mobilize Evidence (HELM) strategy was created as an adaptation of the evidence-based Leadership and Organizational Change for Implementation (LOCI) implementation strategy to enhance elementary school principals' use of strategic implementation leadership to support the adoption and delivery of Tier 1 (i.e., universal social, emotional, and behavioral) EBP. In service of its iterative development, a human-centered design methodology was employed to increase the successful uptake and use of HELM. The Cognitive Walkthrough for Implementation Strategies (CWIS), a novel mixed-methods approach to evaluate implementation strategy usability, was applied to identify and test HELM strategy tasks of critical importance. A sample of 15 elementary school principals participated in group cognitive walkthrough testing as either school principal recipients or HELM coaches. Both user types rated the strategy as acceptable (principal M = 77.8, SD = 15.5; coach M = 87.5, SD = 7.9). Five usability issues were identified using a highly structured common usability issue framework and provided direction for the generation of redesign solutions to be incorporated in a subsequent version of the HELM strategy. The evaluation of the strategy's usability improved its alignment with user needs, expectations, and contextual constraints, rendering a more usable strategy and broadly applicable information surrounding the development of psychosocial implementation strategies in real-world settings. [This is the online first version of an article published in "School Mental Health."]