Military assignment mechanisms provide a unique opportunity to estimate the impact of high-performing mentors on job advancement of their subordinates. Combining US Army administrative data with officer evaluation reports, we find that high-performing mentors positively affect early junior officer promotion and that early promotion probabilities rise as the duration of the high-quality mentorship increases. These effects are largest for high-ability protégés. Junior officers who were exposed to multiple high-performing mentors did not experience an additional increase in promotion rates.