Early postnatal development can have profound effects on life-history traits later in life. One mechanism hypothesized to mediate this relationship is the anabolic hormone, insulin-like growth factor-1 (IGF-1). IGF-1 contributes importantly to postnatal growth, and thus offers a means by which environmental and genetic variation might direct organismal development, reproduction and survival. We tested whether juvenile concentrations of IGF-1 can predict intraspecific variation in life-history traits later in life using longitudinal data from free-living female spotted hyenas (Crocuta crocuta). We found that juvenile concentrations of IGF-1 predicted heavier juvenile mass, which in turn predicted greater survival to reproductive maturity. However, independent of mass, higher juvenile concentrations of IGF-1 predicted earlier age at first parturition and reduced longevity in adulthood. Our results highlight the importance of early postnatal development as a determination period in mammals and suggest that concentrations of IGF-1 during this sensitive period can be used to predict important later-life trade-offs between growth, reproductive fitness and life span in wild, long-lived animals.