Supporting student success in introductory mathematics courses is a growing national imperative in order to both diversify and increase the number of well-prepared Science, Technology, Engineering, and Mathematics (STEM) graduates. Efforts to diversify STEM fields have focused on broadening participation, addressing equitable outcomes, and promoting inclusive learning environments for an array of student identities. At the same time, educational research, institutional programs, and policies to support Queer-spectrum students remain largely underdeveloped and undertheorized. By Queer-spectrum, I mean students who identify as Lesbian, Gay, Bisexual, Transgender, Two-spirit, Intersex, Pansexual, Asexual, or in other ways Queer because of their queer sexual identity or non-cisgender identity (Kumashiro, 2001). Broadly speaking, this dissertation study seeks to explore the lived experience of Queer-spectrum undergraduate STEM students through a transformative mixed methods design (Mayoh & Onwuegbuzie, 2015), which is structured in three phases. Drawing on large scale quantitative survey data (n=25,785) for the first phase, I examine how Queer-spectrum students describe mathematical learning opportunities in introductory mathematics courses and how these reported descriptions differ within Queer-spectrum students and between Queer-spectrum and Straight students. In the second phase of this study, I use a phenomenological approach and grounded theory techniques to identify mathematical discourses (e.g., beliefs, norms) related to Queer identity based on individual interviews with 17 Queer-spectrum students. Based on this analysis, I define the exclusion-irrelevancy space to network together mathematical discourses that positions queer identity as excluded and irrelevant to the pursuit of STEM. In the third phase of this study, I draw on thematic analysis and Nasir’s (2011) identity resource constructs to document the resources that support Queer-spectrum students in STEM using focus groups with Queer-spectrum students at four universities. Taken together, these three phases seek to transform and advocate for inclusive STEM environments for Queer-spectrum students. The aim of this study is to provide both a broad understanding of Queer-spectrum student experiences in mathematics while providing illustrative accounts to capture the nuance of the lived experiences for Queer-spectrum students. I conclude this dissertation study by looking across the three phases and, most importantly, provide implications for practice and policy in STEM education to promote more inclusive STEM environments.