As a child, Jack Darnell lived at Pilot Butte where his father was a ditch rider. While still a toddler they moved to Derby Dome Oil Field where his dad worked as a boiler and at age five, Jack started first grade. When his older sister started high school the family moved to Lander. Jack worked at a local creamery before and after school, but during summer breaks he worked for the Sherlock Ranch on their summer range around South Pass, herding cattle and haying. After graduating Fremont County Vocational High School, at age 17 he joined the U.S. Marine Corps. As a member of the 5th Marine Division, he was on the invasion of Iwo Jima. He was wounded by a Japanese sniper and was awarded the Purple Heart. After recuperating, he returned home to Wyoming and went to work for the Pitchfork Ranch near Meeteetse. He then worked for Husky Oil, before going to work for his cousin, Nate Brown, on his Grass Creek Ranch. Jack would go on to work on other ranches at Hyattville, Glenrock, on the Sweetwater River, and then the Flagg Ranch, where he became manager.