The Smith (pseudonym) School District's mathematics achievement data show disparities among subgroups with respect to middle and high school mathematics learning. Institutional data point to a variety of factors that may indicate low mathematics teaching efficacy and lack of supports for educators, particularly those new to teaching. Because mathematics teaching efficacy impacts teacher instructional practices and student learning, it is important to measure the effect of professional interventions, such as workplace mentoring, on mathematics teachers' teaching efficacy, development, and perceptions. From the perspective of the mentor, this study explored how mentoring impacts mentors' mathematics teaching efficacy and professional development. It followed two veteran (with seven years' or greater classroom teaching experience) middle school mathematics teacher mentors for eight weeks during the 2020 pandemic, as they worked with one or two beginning (with three years' or fewer classroom teaching experience) mathematics teacher mentees. Findings provide a broader framework for improved mentoring between beginning and veteran teachers, and for stronger and more sustained mentoring and mathematics teaching efficacy. When accompanied by district supports, these may lead to improved classroom instruction, as well as improved mathematics achievement for all students within the district and beyond. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]