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.