Learning to teach mathematics content courses designed for prospective elementary teachers (PTs) is challenging. Mathematics content instructors must be prepared to not only teach mathematics content, but engage PTs in understanding children's mathematical thinking strategies, learning trajectories, and misconceptions (Carpenter et al., 1996; Carpenter & Moser, 1982; I et al., 2020). When considering how instructors might best be prepared and supported, however, one must recognize that there is significant variation in the backgrounds and expertise of mathematics content instructors (Masingila et al., 2012; Yow et al., 2016) and little is known about the preparation, knowledge, and experiences of mathematics content instructors (Even, 2008; Goos, 2009; Masingila et al., 2012; Oesterle, 2011; Zaslavsky & Leikin, 2004). Furthermore, most instructors who are newer to teaching PTs do not feel prepared and additionally report an absence of training, resources, and support at their institutions (Goodwin et al., 2014; Masingila et al., 2012; Yow et al., 2016). Mathematics content instructors, especially those newer to teaching PTs, need preparation and support that account for their background and teaching context. My dissertation is a narrative inquiry (Clandinin & Connelly, 2000) of one beginning instructor's experiences learning to teach mathematics for elementary PTs. For one semester, I observed all the instructor's classes and meetings with other instructors, conducted three interviews with the instructor, collected the instructor's teaching and learning autobiographies, and collected the instructor's weekly reflection journals. The findings of my dissertation include narrative themes connecting the instructor's background, preparation, and support to their learning to teach mathematics content for PTs. These findings have numerous implications for the professional development and mentoring of future mathematics content instructors, including how newer instructors might be better prepared or supported to engage in the kinds of newer, more numerous, or more cognitively demanding tasks of teaching associated with mathematics content courses for PTs. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]