He was mentored by the paleontologist Edward Drinker Cope, whom he met on a fossil-hunting expedition in Wyoming . The articulate Fairfield Osborne joined the US Geological Survey in 1900 and became senior vertebrate paleontologist in 1924. He led many fossil-hunting expeditions into the American Southwest, starting with his first to Colorado and Wyoming in 1877, when he met Cope. He described and named Ornitholestes in 1903, Tyrannosaurus rex in 1905, the Pentaceratops in 1923, and the Velociraptor in 1924.
Some of his contributions are less celebrated: Osborn's belief in the now-discredited idea of orthogenesis is one such contribution, his promotion of eugenics, another. Andrews' explorations in the Gobi Desert were in part set in action by Osborn's certainty that the origins of man were to be found in Asia. His unfortunate Man Rises to Parnassus, built on the misleading, but "almost miraculous" Piltdown Man hoax, reveals the deeply-imbedded racism of even the educated classes of his generation, supported on pseudoscience. His (not entirely accurate) description of Hyracotherium as "the size of a small Fox Terrier", has been slavishly copied by many elementary textbooks.
His best known publication might be his two-volume work of 1936, The Proboscidea: A Monograph of the Discovery, Evolution, Migration and Extinction of the Mastodonts and Elephants of the World, in which he discussed the fossil history and evolution of elephants and their relatives. A second volume appeared in 1942, after his death. He published many papers on fossil proboscideans during his career.
Osborn wrote an influential textbook, The Age of Mammals in Asia, Europe and North America (1910). He also authored The Origin and Evolution of Life (1916).
He is also known for his theories on intelligence and racial differences, particularly his work The Evolution of Human Races