This study examines the K-12 mathematical experiences of U.S. university students via an expressive writing assignment: a mathematical autobiography essay. The essays of 67 college students, out of over 300 enrolled in 16 sections of a college liberal arts mathematics course, were analyzed using constant-comparative methods. Two categories of experience connected to aspects of mathematical self-regulation emerged as significant: (1) locus of control for mathematics knowledge and learning; (2) self-evaluations of mathematical ability, efficacy, and potential. Interviews of 18 of the 67 students provided support and clarification for the analysis. An argument grounded in existing research for increased mathematical self-regulation as a result of completing the mathematics autobiography is made. Finally, connections are drawn between learning and psychological theories to support the assertion that using the assignment may help build pedagogical content knowledge among novice college mathematics teachers.