The mentoring of beginning to teachers to work effectively is intuitively important. Yet, there is a lack of research on the effectiveness or impact of structured mentoring, as well as the principles that support effective mentoring. This paper reports on an empirical research using a mixed-method approach to first, quantitatively by means of a survey, measure beginning teachers' perceptions of a structured mentoring package's effectiveness that they underwent, and then qualitatively, through focused group discussions, investigate the underlying principles of effective mentoring in relation to the structured mentoring provisions of the mentoring package. Results from the survey found that the four delivery modes of mentoring (discussions, reflections, reading materials, and feedback on lesson observations) and six content components of the mentoring package (attention, routine, discipline, momentum, space, and time) were perceived by beginning teachers to have positive impact on their classroom management practices. The qualitative findings drawn from focused group discussions showed four underlying principles of effective mentoring: (1) structured-ness, (2) relevance, (3) applicability, and (4) workability.