Background: The Health System Impact (HSI) Fellowship program provides highly qualified post-doctoral fellows studying health services and policy research (HSPR) with opportunities for experiential learning, enriched core competency development and mentorship from senior-level leaders within health system organizations. Its overall aim is to prepare postdoctoral fellows with the research and professional skills, experiences and networks to make meaningful and impactful contributions in careers in academic and applied health system settings. Objective: This study examined whether this HSI Fellowship program has contributed to the development of enriched core competencies in HSPR. Methods: A competency assessment tool was developed and administered to the 46 fellows and their health system and academic supervisors from the inaugural HSI Fellowship cohort. Fellows' self-assessments at baseline, three months and 12 months were analyzed, along with supervisors' assessments at three and 12 months. Descriptive analyses were used to examine competency development over time. Differences by gender and between supervisor and fellow ratings were analyzed. Results: HSI fellows' self-assessments indicate that they strengthened their skills in all 10 enriched core competencies. Supervisors' assessments of the fellows' competencies also improved from baseline to 12 months. Gender differences at baseline disappeared by the 12-month assessment. Conclusion: The HSI Fellowship provides an opportunity to develop the full suite of enriched core competencies, particularly in competency domains that are not currently emphasized in HSPR doctoral curriculum.