One of the important factors influencing the well-being of school counsellors in schools is the mental workload resulting from their job-related tasks. The increasing workload can increase job stress and burnout of school counsellors. In this context, the current study aimed to investigate the mediator role of job stress in the relationship between the mental workload and burnout of school counsellors. To this end, data were collected from 237 school counsellors (151 women). The study revealed that mental workload significantly correlates with job stress and the four sub-dimensions of the counsellor burnout scale, except for the “devaluing client” sub-dimension. In the study, it was found that job stress plays a mediator role in the relationship between mental workload and the sub-dimensions of counsellor burnout. The findings were discussed in light of relevant theories and literature, and some suggestions were made.