This article explores how patterns of engagement and program design impact professional learning and development of mathematics mentor teachers as they participate in an asynchronous mentoring program, e-Mentoring for Student Success (eMSS). In specific, this study: 1) sought to determine if activity level was related to mentors' perceived professional growth; 2) explored the effect of program design on a mentors' perceived growth and over what domains this growth occurred; and 3) sought evidence of the mentor teachers' professional growth in the online dialogue. Results of the study indicate that a mentor's activity level does positively relate to perceived professional growth and that much growth occurs "behind the scenes" and may not be evident in participants' posts. The mentor mathematics teachers reported growth in all assessed domains, particularity in their reflective practices, and attributed their growth to participation in the Content Forum discussion areas more than the other assessed areas. (Contains 5 tables.)