Introduction: Developing physicians as leaders has gained attention across the United States. Undergraduate medical education (UME) and graduate medical education (GME) leader development programs have increased. During postgraduate years (PGY), graduates bring their leadership education to the bedside; however, associations between leader performance in medical school and GME is largely unknown. It is important to find experiences that can assess leader performance that may be useful to predict future performance. The purpose of this study was to determine if (1) there is a correlation between leader performance during the fourth year of medical school versus leader performance in PGY1 and 3, and (2) leader performance during the fourth year of medical school is associated with military leadership performance in PGY1 and 3s while taking previous academic performance markers into account. Methods: This study examined overall leader performance of learners (classes of 2016-2018) during the fourth year of medical school and their graduate leader performance post-medical school. Leader performance was assessed by faculty during a medical field practicum (UME leader performance) and graduate leader performance was assessed by program directors at the end of PGY1 (N = 297; 58.3%) and 3 (N = 142; 28.1%). Pearson correlation analysis examined relationships among UME leader performance and between the PGY leader performance items. In addition, stepwise multiple linear regression analyses were conducted to examine the relationship between leader performance at the end of medical school with military leadership performance in PGY1 and 3, while taking into account the academic performance markers. Results: Pearson correlation analyses revealed that the UME leader performance was correlated with 3 of 10 variables at PGY1, and was correlated at PGY3 with 10 out of 10 variables. Results of the stepwise multiple linear regression analysis indicated that leader performance during the fourth year of medical school explained an additional 3.5% of the variance of PGY1 leader performance after controlling for the previous academic performance markers (MCAT total score, USMLE Step 1 score and Step 2 CK score). In contrast, leader performance during the fourth year of medical school alone accounted for an additional 10.9% of the variance of PGY3 leader performance above and beyond the set of academic performance markers. Overall, UME leader performance has more predictive power in PGY leader performance than the MCAT or USMLE Step exams. Conclusions: The findings of this study indicate that a positive relationship exists between leader performance at the end of medical school and leader performance in PGY1 and 3 years of residency. These correlations were stronger in PGY3 compared to PGY1. During PGY1, learners may be more focused on being a physician and an effective team member compared to PGY3 where they have a deeper understanding of their roles and responsibilities and can take on more leadership roles. Additionally, this study also found that MCAT and USMLE Step exams performance was not predictive of PGY1 or PGY3 leader performance. These findings provide evidence of the power of continued leader development in UME and beyond.