Dennis L. Mangrum grew up in Salt Lake City. He attended West High School in the days when if you didn't beat them during the game, you did after. His English teachers considered him a jock who dabbled in academics. He attended the University of Utah on a football and wrestling scholarship and continued to dabble in academia, graduating with a master's degree in civil engineering. He later attended law school at the University of Utah and then practiced law in California for twenty years before returning to Utah.

While practicing law in Utah, he was also the pitching coach at Brighton High School. He coached his son Micah, who pitched for BYU and went on to pitch professionally for seven years. He also coached his son Shane, who went on to play for Harvard. His youngest son is now playing for Brighton, with his dad coaching from behind the fence. Dennis and his wife live in Sandy, Utah. A baseball junkie and a die hard Red Sox fan, the bumper sticker on his car reads, "I Don't Brake for Yankee Fans."