Marine Corps instructors typically serve three-year assignments with no prior teaching experience. Although they may be subject-matter experts, the ability to pass knowledge to others is a distinct skill set and the training they receive to do so varies greatly. To maximize instructor performance, there is a need to accelerate the development of their teaching proficiency. To address this need, our team developed a mastery model for USMC instructors which describes the desired performance and provides a roadmap for development (Vogel-Walcutt, Phillips, Ross, & Knarr, I/ITSEC, 2015). The model was adopted as the basis for a new Training and Readiness Manual for instructors and as part of staff and faculty development policy. This paper reports on the next step, application of the model to develop and validate a Marine Instructor Assessment Toolkit. Assessment tools were developed to support the formal schools in accelerating instructor development with feedback-oriented instruments. They include an Observation Rubric for instructional settings, a Supervisor Rating Form for holistic instructor performance, a Self-Reflection Tool, and a Situational Judgment Test. The tablet- and web-based tools were field tested to gather user input at formal schools, and data were subjected to psychometric analysis which found the Observation Rubric and Supervisor Rating Form to be reliable and valid instruments. After field testing, the tools were finalized based on the psychometric analysis and user input. As the front end of transition to the formal schools, a baseline of instructor proficiency is currently in process to include a sample of up to 300 instructors. The baseline will serve as comparison data for future instructor performance measurements after full implementation of the instruments across the formal schools. Transition efforts following establishment of the baseline will consist of train-the-trainer workshops to familiarize the formal schools with the mastery model and the assessment tools.