There is a growing interest in ensuring that students are prepared for college and a career when they finish high school. Research identified several indicators that are associated with college preparedness including attendance, grade point average (GPA), and test scores. However, it is difficult to use these indicators to determine the preparedness level of the nation's students in general. This study used results from National Assessment of Educational Progress (NAEP) mathematics assessment, the "gold standard" assessing what students know and can do in grade 12 to establish college preparedness benchmarks. This study linked ninth-grade student background data and school-reported high school and postsecondary transcript data from the National High School Longitudinal Study of 2009 (HSLS:09) to student item responses on the 2013 National Assessment of Educational Progress (NAEP) mathematics assessment to examine the relationship between college preparedness and NAEP mathematics achievement. NAEP scale scores imputed for all HSLS:09 participants via marginal maximum likelihood regression analyses were used to predict college preparedness. This study extended the earlier investigations of college preparedness based on NAEP grade 12 mathematics data from the prediction of college enrollment alone to the prediction of additional postsecondary outcomes including remedial course-taking and first-year college GPA based on postsecondary transcript data. Defining college preparedness as enrolling in a 2- or 4-year college without remedial course-taking and acquiring a first-year college GPA of 2.7 or higher as did National Assessment Governing Board, this study showed that the probability of college preparedness increased from 26 percent for students performing at the NAEP Basic level to 58 percent at the NAEP Proficient level and 87 percent at the NAEP Advanced level. The probabilities at preparedness increased when the definition of college preparedness omitted remedial course-taking. Although similar patterns were observed for various student groups including race/ethnicity, gender, and socioeconomic status, there were some differences in results across groups. Since the report shows the probability of college preparedness (under various definitions) at each NAEP achievement level, it is thereby also providing important validity information for these NAEP achievement levels.