This study investigated the relationship between preservice elementary mathematics teachers’ beliefs about mathematics and their mathematical modeling competencies. In the study, the belief categories of the preservice teachers were first determined using Q methodology and then classified into traditional and non-traditional belief. A Mathematical Modeling Competencies Rubric was developed in line with the literature and expert opinions. Three independent experts used this rubric to evaluate holistic modeling tasks that the participants completed. The resulting scores were analyzed using the Many-Facet Rasch Model to test for differences in modeling competencies among the belief groups. The findings revealed that preservice mathematics teachers with non-traditional beliefs demonstrated higher modeling competencies than those with traditional beliefs (χ2 = 84.7, df = 3, p < 0.001). In conclusion, the study highlights that preservice mathematics teachers’ beliefs about mathematics play a crucial role in developing modeling competencies and suggests that belief structures should be considered in teacher education programs.