This paper reports a study of 10 post-secondary STEM (Science, Technology, Engineering, Mathematics) instructors' beliefs about mathematical modelling and the role of mathematics in STEM coursework. The participants were selected from STEM disciplines that are atypical to the literature base (e.g. anthropology and geography), in order to extend what is known about STEM instructors' beliefs to other disciplines. We conducted episodic narrative interviews to hypothesise the genesis of participants' most salient beliefs. We then conducted a cross-case synthesis to reflect on the similarities between our participants' beliefs and findings previously reported in STEM education literature. Our participants held many beliefs in common with typical STEM instructors with regards to how they define modelling, the role of modelling in STE (Science, Technology, Engineering) courses, and their views of students as learners of mathematics and modelling. Our analysis suggests participants' commitments within these categories are interdependent and arise from lived experiences. Additionally, participants within the same field held competing beliefs about modelling, suggesting that constituting 'major' as an independent variable in future research may not be straightforward.