In apes and humans, neutrophil to lymphocyte ratio (NLR) can be used as a predictive indicator of a variety of clinical conditions, longevity, and physiological stress. In chimpanzees specifically, NLR systematically varies with age, rearing, sex, and premature death, indicating that NLR may be a useful diagnostic tool in assessing primate health. To date, just one very recent study has investigated NLR in old world monkeys and found lower NLR in males and nursery-reared individuals, as well as a negative relationship between NLR and disease outcomes. Given that baboons are increasingly used as research models, we aimed to characterize NLR in baboons by providing descriptive data and examinations of baboon NLR heritability, and of the relationships between NLR, age, rearing, and sex in 387 olive baboons (Papio anubis) between 6 months and 19 years of age. We found that (1) mother-reared baboons had higher NLRs than nursery-reared baboons; (2) females had higher NLRs than males; and (3) there was a quadratic relationship between NLR and age, such that middle-aged individuals had the highest NLR values. We also examined NLR as a function of transport to a new facility using a subset of the data. Baboons exhibited significantly higher transport NLRs compared to routine exam NLRs. More specifically, adult baboons had higher transport NLRs than routine NLRs, whereas juveniles showed no such difference, suggesting that younger animals may experience transport stress differently than older animals. We also found that transport NLR was heritable, whereas routine NLR was not, possibly suggesting that stress responses (as indicated in NLR) have a strong genetic component. Consistent with research in humans and chimpanzees, these findings suggest that NLR varies with important biological and life history variables and that NLR may be a useful health biomarker in baboons.