Teacher well-being is an important issue that needs to be considered within a teaching environment. However, little research exists about the relationship between teacher well-being and learner performance. In this study we used data from the 2015 Trends in International Mathematics and Science Study (TIMSS) to look at the interplay between teacher-related factors (including stress), and the mathematics achievement of Grade 9 learners from South Africa in the 2015 TIMSS. The results show that levels of teacher stress, teacher qualification, class size, as well as the type of area in which the school is situated are all significantly associated with learner achievement. The findings of this study reveal that learners who are taught by teachers who feel more stressed, have large classes to contend with, are in schools located in poorer areas, and who are less qualified, are significantly more disadvantaged with regard to their mathematics education than their counterparts who are taught by teachers who do not share these problems. These findings suggest that it is important to consider the role of positive working environments for teachers in trying to find ways to improve the mathematics learning outcomes.