Karl Castleton grew up in the mountains of Colorado, in Silverton and Telluride, where he was first introduced to Apple and TRS-80 computers. He spent much of his time in junior high school in the library, learning to program those computers. He graduated from Central High School in Grand Junction in 1985. Castleton took classes at CMU during that time in electronics, as part of a vocational tech program.
Castleton started as a student at CMU who only wanted to get more education than a high school diploma. While taking electronics technology classes, he discovered he enjoyed programming and earned an AAS and began his bachelor's degree in computer science. Near the end of his work on the computer science degree, Castleton realized it was not much more effort to complete a mathematics degree as well. After graduating in 1992 with both degrees, he was hired by a national laboratory to do systems integration work. He earned a Master's degree at Washington State University during his time at the national laboratory. His career with the lab lasted 20 years.