Recognizing that post-secondary student attainment and achievement is of increasing import for economic success, this study compares the post-secondary performance of students who attended public and private high schools. The results generally indicate that students who attended Catholic high schools had higher college grade-point averages (GPAs), were more likely to graduate, and were more likely to graduate with a STEM degree. This Catholic school advantage was wide-ranging, benefiting many subgroups of students, including non-white, low income, urban, and low-achieving students. We controlled for a rich set of factors and conducted sensitivity analyses to establish the strength of these results.