The current student achievement gap can be attributed, in part, to the perceived and actual shortage of highly qualified principals prepared to be effective instructional leaders (Kearney, 2010). Most school districts within do not offer consistent targeted professional development programs for mid-career principals that will develop principals' skills in the supervision of instruction. This study was designed to examine how a mentor-coaching model could impact mid career principals in the refinement of supervision skills. This qualitative Action Research study examines the impact that mentor-coaching strategies have on refining the instructional supervision skills of Developing (mid-career) Principals. Five Mentor Coach Principals used Blended Coaching strategies to mentor and coach 10 Developing Principals. Qualitative data collected from journals, interviews, focus groups, questionnaires and open-ended surveys was used to measure the impact that Mentor-Coaches had on the Developing Principals when they worked with their teachers during pre and post lesson observation conferences. Mentor-coach and Developing principals perceived that active listening, asking probing questions, providing immediate actionable feedback to teachers, following the conventions of the clinical supervision model, establishing a focused lens for lesson observation, and developing trusting relationships are integral elements having the greatest impact on the supervision of instruction. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]