Purpose:To investigate the ethnicity of Kenya’s most successful international runners, tracking their evolution over the period of their international emergence and current dominance.Methods:The authors analyzed male track distance events from 800m upwards from all the major global athletics championships from 1964 to 2013, and the annual Top-25 world marathon performances since 1990.Results:The percentage of top-25 marathon performances and medals won by Kenyan and Kalenjin runners have increased over time with Nandi subtribe outperforming the rest of the world outside Africa (r > .70, large effect). However, Europe, North America, Oceania, Asia, and South America decreased over time in top marathon performances and track medals won (r > .70, large effect). The tribe and subtribe distribution was different in the marathon than in the track: Maasais were more likely to feature in medals won in shorter track events than in the top 25 of the world marathon rankings (risk ratio [RR] = 9.67, very large effect). This was also the case for Marakwets (RR = 6.44, very large effect) and Pokots (RR = 4.83, large effect). On the other hand, Keiyos, Kikuyus, Kipsigis, Sabaots, and Tugens were more likely to succeed in the marathon than in shorter track events (RR > 2.0, moderate effect).Conclusion:These data emphasize that the previously documented emergence of African distance runners is primarily a Kenyan phenomenon, driven by the Kalenjin tribe and in particular the Nandi subtribe. This supports the complex interaction between genotype, phenotype, and socioeconomic factors driving the remarkable dominance of Kenyan distance runners.