This study applies life course theory (Elder, 1998) to understand the effect of different mathematics course progressions throughout the high school transition period. The purpose of the study was two-fold: to investigate the effectiveness of two different high school’s mathematics course progressions by examining mathematics achievement throughout the high school transition; and to relate students’ experiences of their mathematics trajectory, formed by their mathematics course progression, to their transition to high school and throughout high school. Using mixed methods, achievement data from eighth to eleventh grade was evaluated from two Alabama high schools in the same school district. Independent samples t-tests, linear regression models, ANOVA, and ANCOVA were performed followed by interviews of three students from each school. Findings from this study indicate that there were significant differences in achievement at the two high schools at the beginning of the transition period and no significant differences in achievement at the end of the transition period. Significant predictors of achievement include prior achievement and the number of courses students took during the transition period. Furthermore, through student interviews factors that influenced student achievement were exposed. This study connects high school transition research with high school mathematics achievement research, and contributes to the lack of qualitative research of the high school transition.