Our study aimed to examine the transformation of pre-service mathematics teachers' theoretical knowledge into in-service mathematics teachers' instructional practice. We carried out seven focus group interviews, three individual phone call interviews, and classroom observations in four semesters with three Turkish pre-service mathematics teachers. This longitudinal study provides a significant insight into the transformation of knowledge through three phases: (i) theoretical knowledge (pre-service teacher education); (ii) practices in internships (pre-service teacher education); and (iii) real classroom settings (in-service teaching experience). The participants realised the difficulties related to implementing student-centred approaches when they experienced real classroom settings during their internships. Despite the participants' rigorous knowledge of teaching and motivation, their realisation about practice was changed through the experiences they had during their internships. Upon becoming in-service mathematics teachers, the participants encountered a need to adapt student-centred activities according to their student and classroom circumstances.