The biogeography of fungi is poorly understood. A species in a novel location may be an introduction, or an endemic newly identified within its native range. The fungus Amanita phalloides is deadly poisonous and a rich literature records the history of this notorious mushroom in America. Although it was first reported from California in 1880, it is often described as introduced. The contradiction within the literature reflects a general confusion about endemic ranges of microbial species. Using historical sources and newly developed molecular markers as complementary data, we establish A. phalloides as an introduction currently expanding its range on the West Coast of North America. We trace its history in California by combining a molecular annotation of herbarium accessions with information about its current range. With these data we can calculate the speed with which A. phalloides reaches novel habitats, as well as the range of its host associations.