This paper explores the complex evolution of the role anthropologists have played as cultural experts in the regulation of the entheogenic use of the peyote cactus throughout the 20th century. As experts of the “peyote cult”, anthropologists provided testimonies and cultural expertise in the regulatory debates in American legislative and judiciary arenas in order to counterbalance the demonization and prohibition of the medicinal and sacramental use of peyote by Native Americans through state and federal legislations. In the meantime, anthropologists have encouraged Peyotists to form a pan-tribal religious institution as a way to secure legal protection of their practice; in 1918, the Native American Church (NAC) was incorporated in Oklahoma, with its articles explicitly referring to the sacramental use of peyote. Operating as cultural experts, anthropologists have therefore assisted jurists in their understanding of the cultural and religious significance of peyote, and have at the same time counseled Native Americans in their interaction with the legal system and in the formatting of their claims in appropriate legal terms. This complex legal controversy therefore provides ample material for a general exploration of the use, evolution, and impact of cultural expertise in the American legal system, and of the various forms this expertise can take, thereby contributing to the contemporary efforts at surveying and theorizing cultural expertise. Through an historical and descriptive approach, the analysis notably demonstrates that the role of anthropologists as cultural experts has been marked by a practical and substantive evolution throughout the 20th century, and should therefore not be restrictively understood in relation to expert witnessing before courts. Rather, this paper underlines the transformative and multifaceted nature of cultural expertise, and highlights the problematic duality of the position that the two “generations” of anthropologists involved in this controversy have experienced, navigating between a supposedly impartial position as experts, and an arguably biased engagement as advocates for Native American religious rights.