Hartley was born on May 19, 1881, in Milton, Wisconsin, to English parents, Harrad and Mary Thompson Jackson. His interest in natural history developed at an early age, and he started a collection of birds at age 11. By the time he was 14, his interest began to focus on mammals, which remained his primary interest throughout the rest of his life. His first publication, which was on screech owls, appeared in Nidologist in 1897, when he was 16. His first mammal paper, which dealt with the meadow voles of Wisconsin, appeared in 1903 in the Milton College Review.
Hartley received his BS degree from Milton College in 1904 with majors in zoology and chemistry. While a student he played quarterback on the football team. After graduation he taught in the Science Department of Carthage Collegiate Institute in Missouri. Two other science teaching jobs followed in short order at Juda, Wisconsin, and Waukegan, Illinois. He entered the University of Wisconsin in 1908 for graduate studies and obtained his MA degree in 1909. While there he identified and catalogued the Department of Zoology bird collection. He later donated his own mammal collection to that institution.
On February 16, 1910, he joined the research staff of the Bureau of Biological Survey, U.S. Department of Agriculture, in Washington, D.C., where he was put in charge of its mammal collection. During the early years of his government employment, he continued his work toward a PhD degree in zoology, which he received from George Washington University in 1914. Hartley continued regular field work in Wisconsin until 1922, and the results appeared as a well-illustrated book, The Mammals of Wisconsin, in 1951. In the introduction he defended the Life Zone concept of C. Hart Merriam and his other Biological Survey associates against the then current trend of criticism by ecologists. He used that concept as a basis for organizing plant and animal distribution in Wisconsin in three life zones.
Hartley wrote two issues of North American Fauna--no. 38 in 1915, A review of the American Moles, and no. 51 in 1928, A taxonomic review of the American Long-tailed Shrews. The latter probably was his most important mammalogical paper because, with minor modifications, the classification that he proposed is still in use today.
After a series of research supervisory positions beginning in 1924, he was made chief of a new unit in the Division of Wildlife Research, which in 1936 became known as the Section of Wildlife Surveys. Research in that unit included life history, taxonomic, distributional, and ecological studies of wildlife, game management planning surveys, cooperative wildlife research in land grant colleges, investigation of wildlife resources in Alaska, and research on National Forest wildlife.
Next to the American Society of Mammalogists, for which he felt a paternal interest as chairman of the founding committee, Hartley probably enjoyed most his affiliation with the Washington Biologists’ Field Club. He was elected to membership in 1925, and served as president from 1945 to 1948. Hartley also was president of the Biological Society of Washington from 1931 to 1933. He was a member of the American Association for the Advancement of Science, the American Ornithologists’ Union, Cooper Ornithological Society, Ecological Society of America, Washington Academy of Sciences, and several scientific societies in Wisconsin.
Hartley retired in 1951, after 41 years of government service, and died at the age of 95 in September of 1976 at his retirement home in Durham, North Carolina.