The twofold purpose of this study was to trace prospective and practicing mathematics teachers' understandings of content area reading instruction in relation to domain knowledge in mathematics, and to examine the extent to which online pedagogical mentoring supported the integration of such instruction and knowledge. The design called for two pairs of prospective and practicing mathematics teachers to develop, implement, and reflectively evaluate four lessons. A multilevel mentoring approach was used to leverage the valuing of domain knowledge in mathematics. Course materials, lesson plans, teacher reflections, mentors' feedback, interviews, and case reports were analysed using Bourdieu's concepts of cultural capital, field, and misrecognition. Results indicate that despite the study's focus on prioritising domain knowledge through pedagogical mentoring, the instances in which such knowledge were integrated effectively with reading instruction varied in relation to a mentor's expertise in mathematics. If reading teacher educators are to support prospective and practicing mathematics teachers in content area reading instruction, additional sources of mathematics cultural capital are needed.