Domestic dogs (Canis lupus familiaris) are the most variable-sized mammalian species on Earth, displaying a 40-fold size difference between breeds. Although dogs of variable size are found in the archeological record, the most dramatic shifts in body size are the result of selection over the last two centuries, as dog breeders selected and propagated phenotypic extremes within closed breeding populations. Analyses of over 200 domestic breeds have identified approximately 20 body size genes regulating insulin processing, fatty acid metabolism, TGFβ signaling, and skeletal formation. Of these, insulin-like growth factor 1 (IGF1) predominates, controlling approximately 15% of body size variation between breeds. The identification of a functional mutation associated with IGF1 has thus far proven elusive. Here, to identify and elucidate the role of an ancestral IGF1 allele in the propagation of modern canids, we analyzed 1,431 genome sequences from 13 species, including both ancient and modern canids, thus allowing us to define the evolutionary history of both ancestral and derived alleles at this locus. We identified a single variant in an antisense long non-coding RNA (IGF1-AS) that interacts with the IGF1 gene, creating a duplex. While the derived mutation predominates in both modern gray wolves and large domestic breeds, the ancestral allele, which predisposes to small size, was common in small-sized breeds and smaller wild canids. Our analyses demonstrate that this major regulator of canid body size nearly vanished in Pleistocene wolves, before its recent resurgence resulting from human-imposed selection for small-sized breed dogs.