Nelson was born in Manchester, New Hampshire, on May 8, 1855, the first son of William and Martha Nelson. Nelson and his brother then lived with his maternal grandparents in the Adirondacks when his father joined the Union Army and mother went to Baltimore as a nurse. Here he fell in love with the wilderness. Nelson moved to Chicago after his father was killed in the Civil War and his mother established a dressmaking business. In 1871, his large insect collection was lost in the Chicago Fire and the family was left homeless. This was the time that he moved focus from insects to birds. He went to Cook County Normal School from 1872 to 1875 where the principal, W. W. Wentworth encourage him. Nelson also met Henry Henshaw and Edward Drinker Cope who helped him develop his interests in birds. In 1877. Nelson joined the U.S. Army Signal Corps. Spencer Fullerton Baird was responsible for selecting Signal Officers for the remoter stations, and would choose men with scientific training who were prepared to study the localflora and fauna. Baird sent Nelson to St. Michael, Alaska. Nelson was the naturalist on board, which sailed to Wrangel Island in search of the Jeannette expedition in 1881. Nelson published his findings in the Report upon Natural History Collections Made in Alaska between the Years 1877–1881. He also published his ethnological findings in The Eskimo about Bering Strait. In 1890 Nelson accepted an appointment as a special field agent with the Death Valley Expedition under Clinton Hart Merriam, chief of the Division of Ornithology and Mammalogy, United States Department of Agriculture. After this expedition he was ordered to conduct a field survey in Mexico, and Nelson remained in the country for the next fourteen years. Nelson continued to work for the Bureau of Biological Survey until 1929, being chief of the bureau from 1916 to 1927. The desert bighorn sheep and Nelson's milksnake were named in his honor. The holotype of the milksnake was collected by Nelson and Edward Alphonso Goldman on July 18, 1897. He worked with Goldman for ten years surveying Mexican terrestrial vertebrates. Nelson's sparrow was also named for him. Rodents named in his honor include Oryzomys nelsoni, Xenomys nelsoni, Ammospermophilus nelsoni, Heteromys nelsoni, Dicrostonyx nelsoni, Dipodomys nelsoni, Chaetodipus nelsoni, Megadontomys nelsoni, Neotoma nelsoni and Nelsonia. In addition to Nelson's milk snake, four other reptiles are named in his honor: Nelson's anole, Nelson's tree lizard, Nelson's spiny lizard, and Nelson's spotted box turtle. He was the president of the American Society of Mammalogists from 1921 to 1923. He also served as president of the American Ornithologists' Union and the Biological Society of Washington. He never married.
