A teaching practicum is a course of study in which pre-service teachers get to experience actual teaching in real classrooms. Mentor teachers who are assigned to mentor and supervise pre-service teachers have many important roles to play in the practicum experience, yet no extensive research has been conducted on these roles. This study sought to determine the roles played by mentor teachers in pre-service teachers’ teaching practicum. Using an explanatory, sequential, mixed-methods research design pertaining to the Malaysian context, we recruited 385 pre-service teachers who had attended teaching practicum and 6 mentor teachers who had previously mentored pre-service teachers. Online questionnaires and telephone interviews were used sequentially. Findings showed that mentor teachers played moderate roles in mentoring pre-service teachers; mentor teachers perceived themselves to play many roles yet regarded certain roles as unnecessary and unimportant. Universities, secondary schools, and the Malaysian Ministry of Education must address the importance of producing quality teachers by intervening as early as pre-service teachers’ teaching practicum.