This prospective, mixed-method study investigates the development of school-based mentoring relationships using direct observations, in-depth interviews, and questionnaires from the perspective of mentors and students. A pattern-oriented analysis of qualitative data explores the diversity observed in the life-course of mentor-student relationships. Systematic variation in developmental trends across relationships revealed four distinctive groupings. Some relationships showed progressive improvement in strength and quality. Others started well but reached a plateau and did not become particularly close. A third group struggled throughout to make a connection. Finally, some relationships succeeded after a breakthrough to overcome their challenges. These inductively derived categories are corroborated and supplemented with quantitative data regarding relationship quality. The study reveals the heterogeneity of relationship trajectories within school-based mentoring programs.