Many pre-service teachers suffer from mathematics anxiety, which hinders their mathematics learning. This can cause a deficit in their mathematics content knowledge, which must be addressed to become effective mathematics teachers. The present study explored whether positive expressive writing was better at mitigating pre-service teachers' mathematics anxiety than general expressive writing. Expressive writing is emotional writing, which was shown to be successful in medical fields to reduce anxiety and more recently in the field of education. In contrast, research on positive expressive writing has recently shown beneficial results for treating test anxiety. Therefore, the potential for positive expressive writing in reducing mathematics anxiety was examined in this mixed methods study involving 27 pre-service teachers enrolled in a hybrid (i.e., half of the instructional time was in-person, while the other half was online) mathematics for teachers course. Participants were randomly assigned to one of three writing groups (positive expressive, expressive, and neutral) and completed pre- and post- measures of mathematics anxiety using the abbreviated mathematics anxiety rating scale at time 1 (beginning of course), time 2 (mid-term examination), and time 3 (final examination). In addition, participants completed a measure of their mathematics ability at the beginning of the course and then proceeded to engage in the three types of writing at the beginning of each of the nine consecutive classes. Although results did not show a significant difference between the three writing groups (positive, expressive, and neutral) and mathematics anxiety, there was a significant relationship between pre-service teachers' mathematical ability and measure of mathematics anxiety taken at the mid-term examination. This strong negative relationship demonstrated that pre-service teachers with low mathematics ability have high mathematics anxiety. Valuable insights were gained into mathematics anxiety effects on pre-service teachers, as well as contextual factors in terms of writing.