In support of the Marine Corps Vision and Strategy 2025 task to "improve small unit leader intuitive ability to assess, decide, and act in a more decentralized manner," the Training and Education Command (TECOM) initiated a Small Unit Decision Making (SUDM) program to improve the cognitive agility and readiness of the force. The SUDM initiative seeks to harness the vast amounts of expertise resulting from the past decade of combat to instantiate training and assessment strategies that accelerate the development of small unit leader decision expertise, especially in light of demands for individuals who can adapt across the range of current and future missions and environments. A fundamental requirement of the SUDM objective is a description of the gold standard for the small unit leader and the progression to mastery of that standard. This paper reports on the construction and application of a squad leader mastery model that has become the foundation for Marine Corps efforts to reduce the variation in infantry forces and purposefully design instructional interventions across the training and education continuum that target higher order cognitive competencies. The mastery model has its theoretical underpinnings in a five-stage model of cognitive skill acquisition. Researchers applied three knowledge elicitation techniques with 58 Marine subject-matter experts. Data analysis resulted in nine key performance areas, and profiles and behavioral indicators of each of five stages of cognitive skill development within each performance area. Incidents from the data supported operational definitions of 15 cognitive constructs hypothesized to enable decision making. The mastery model is informing development of cognitive readiness training by other researchers supporting Marine initiatives, and was applied to create a behaviorally anchored rating scale for assessing domain mastery as part of the development of a larger SUDM assessment battery.