While substantial research has shown the relationship between earlier mathematics coursetaking and later mathematics achievement, recent research suggests that mathematics motivation is an important predictor of grade 12 mathematics achievement as well. The current study extends this research by examining the role of mathematics identity at grade 11 for grade 12 mathematics achievement, and whether differences in self- versus other- appraisals of whether a student is "a mathematics person" are related to mathematics achievement after taking mathematics motivation into account. We use a unique dataset from a sample of U.S. students who participated in both the High School Longitudinal Study of 2009 and grade 12 National Assessment of Educational Progress mathematics assessment. The analyses reveal that the stronger the students' mathematics identity the better their grade 12 mathematics achievement even after accounting for students' mathematics and science course-taking patterns, their STEM grade point average (STEM GPA), prior mathematics achievement (measured at grade 9) and background characteristics. While initial analyses suggested that higher mathematics achievement is also associated with perceived appraisals by others being more positive than self-appraisals with respect to being "a math person," that relationship was found to be non-significant when other variables in the model are included. The theoretical implications of the study's findings are discussed.