Mills was born in Kansas but moved to Colorado early in his life during a bout with tuberculosis. At age 15, he made his first ascent of the 14,255-foot Longs Peak. Over the course of his life, he made the trip 40 times by himself and nearly 300 additional times as a guide.
In 1887, after returning to health, he moved to Butte, Montana. There he lived and worked intermittently until 1902, spending more summers traveling the West Coast of the United States, Alaska, and Europe. In 1889, he had a chance encounter with famed naturalist John Muir on a San Francisco beach, and from that point on Mills dedicated his life to conservation activism, lecturing, and writing.
In 1902, Mills returned to Colorado and purchased from his cousin the Longs Peak House in Estes Park. He eventually homesteaded in the surrounding area and later turned the Longs Peak house into the Longs Peak Inn, from which he treated guests to excursions into the wilderness and evening nature talks.
From 1902–1906, Mills also served as the Colorado State Snow Observer, a position that took him into the wild he so loved. His job was to measure the snow depths to predict spring and summer runoff. Following this position, he served as government lecturer on forestry from 1907–1909. During this time, he also authored several articles and books on nature and Estes Park area.
Throughout his time in various assignments, Mills was also leading the fight to preserve the area around Longs Peak as a national park. Aided by groups such as the Sierra Club and the Daughters of the American Revolution, Mills succeeded and Congress established Rocky Mountain National Park in 1915. Called the "Father of Rocky Mountain National Park," Mills continued to lecture and write books until his death at age 52 in 1922. Mills Lake in Rocky Mountain National Park is named in his honor.
- Adventures of a Nature Guide'
- The Grizzly, Our Greatest Wild Animal
- In Beaver World
- Rocky Mountain Wonderland
- Story of a Thousand-Year Pine
- Wild Life on the Rockies
- Story of Estes Park, Grand Lake, and Rocky Mountain National Park
- Story of Scotch
- Spell of the Rockies
- Waiting in the Wilderness
- Watched By Wild Animals, Doubleday, Page & Company, 1922.
- Your National Parks
- Rocky Mountain National Park Dayhiker's Guide, Jerome Malitz, Big Earth Publishing, 2005.
- Wild, Peter (July 29, 1977). "The Father of Rocky Mountain Park: Enos Abijah Mills". High Country News (Launder, WY): 1, 6–7.
- Wild, Peter (1979). "Enos Mills: Propagandist of the Rocky Mountains". Pioneer Conservations of Western America. Missoula, MT: Mountain Publishing. pp. 71–79.