John Stewart Day was born in 1909 and grew up in Medford, Oregon. His father was judge and state representative Earl Benjamin Day, and his younger brother Ben also served in the Oregon Legislature. As a young man, John S. Day worked at Crater Lake National Park, and he studied at Oregon State College (later Oregon State University); the University of Oregon; and the Babson Institute in Wellesley, Massachusetts.

Day co-owned the Blue Moon Ranch near Medford, Oregon, with movie star Ginger Rogers. In the late 1940s, he made efforts to create a Medford-area resort, Table Rock Estates, for celebrities. While this project faltered, another of his real estate developments, Verde Hills, was successful and incorporated into the city of Medford.

Day was athletically inclined throughout his life, playing football in high school, and going on big game hunting expeditions as an adult. His athletic activities took on a new importance when, at about age 40, he was diagnosed with arthritis. Doctors told him that he could no longer ride a horse, but did encourage him to walk and lift weights. This spurred Day’s mountaineering career, during which he set a number of mountain-climbing records in North America. In 1960, he attempted a climb on Mt. McKinley (later Denali), but this ascent ended in tragedy when his party experienced a fall, injuring Day and killing one of his fellow climbers. One of the rescue planes sent for them also crashed, killing two more people.

In the 1960s, Day began to ski competitively. He founded the Oregon Nordic Club, and was vice president of the Pacific Northwest Ski Association in 1967. He was also involved with the United States Ski Association, the United States Ski Education Foundation, and the United States Ski Team. He was an avid promoter of ski touring. Day was also active in bicycling and canoeing, and remained physically active late into his life.

In 1935, Day married Mary Brown Parsons, the daughter of businessman Reginald H. Parsons; they had one son, Jack. Day was also a member of the Explorers Club and the Adventurer’s Club. John S. Day died in his Medford home in May 1986, just a few months after his wife’s death.