In the first four decades of the twentieth century, horse racing was one of America’s most popular spectator sports. Members of America’s elite took to breeding and racing horses as one of their preferred pastimes. Coinciding with an increase in immigration and the rediscovery of Mendelian genetics, the idea that careful breeding of thoroughbreds would result in improved horses resonated with Americans worried about racial degeneration. Scientists committed to racial ideologies looked to thoroughbreds—whose owners and breeders maintained extensive pedigree records—to understand the science of genetic inheritance. Harry H. Laughlin, superintendent of research at the Eugenics Record Office at Cold Spring Harbor, Long Island, pored over breeding charts and race results to develop a mathematical model of inheritance that he called the “inheritance coefficient.” He believed his careful study of horses would yield findings that he and his fellow eugenicists could apply to humans. Thoroughbred breeders followed trends in genetics while contributing to the production of scientific knowledge. Pedigree charts available to bettors at race tracks helped normalize concepts of biological inheritance for race track attendees of all classes. Horse racing’s popularity in the United States contributed to the diffusion of the concept of biological race that originated as an ideological project of the ruling class. This paper analyzes the role of thoroughbred breeding and racing in the formation and popularization of racial ideology by situating breeding farms as sites of knowledge production and racecourses as places that exhibited performances of racial science for large audiences.