This study aims to better understand the role of mentor teacher-mediated experiences in preservice teachers (PTs)' progress toward the vision of teaching advocated by their programs. Data were collected from multiple cohorts of preservice science teachers at two university-based teacher preparation programs. Employing a qualitative, multiple case study approach, a total of 35 cases were analyzed focusing on the quality of mentor teacher-mediated experiences (i.e., modeling program-advocated vision of teaching, supporting PTs' experimentation, and providing feedback), and its relationship to PTs' progress over time. The analyses show that mentor teachers' supportiveness for PTs' experimentation played a critical role in facilitating PTs' desirable changes. Well-structured experimentation created conditions for PTs to notice, leverage, and expand students' sense-making repertoires in classrooms. Mentors' modeling of program-recommended practices was not necessarily related to PTs' progress. This study raises questions about prevalent perceptions of a good mentor teacher as someone who models program-recommended practices.