He felt a strong sense of unfinished business a player for US national and Olympic teams, having never won a medal. It gave him a chip on his shoulder, despite success as a college coach.
He went from working at an insurance agency and playing amateur hockey to coaching the University of Minnesota, first as an assistant, then as Head Coach, beginning in 1972.
He began to establish his underdog coaching credentials by winning the NCAA Division I Championship in his second year, 1974. But what really gives him "Doggie" cred is that he was the first to win this title with a team exclusively made up of U.S. born talent.
And then, six years later, though his methods were questioned by players and even his assistants, Brooks pulled the ultimate underdog-rabbit out of the hat by coaching the U.S. Olympic team, a team of amateurs, to a Gold Medal against the Soviet team of professionals—what could be considered the greatest upset in the history of sport.
There is no doubt that Brooks is the only coach that could have achieved this. There may be a few coaches who have personal stories that are more impressive from an underdog perspective, but the combination of Brooks' story of unfulfilled Olympic dreams, his insurance-agent day job, and his achievements at University of Minnesota and ultimately the Miralce on Ice make him the greatest Underdog Coach.
Note: Yes, this is a list of professional athletes, so how does Brooks qualify? He finished his career coaching in the NHL even though his greatest underdog achievements were in the amateur ranks. Of course even in college and The Olympics he was a professional coach, and in the Olympics he coached his team against a professional team. Finally, had it not been for his success in the amateur realm, he never would have become a coach in professional hockey. Thus, he qualifies, especially since it's my list and I make and interpret the rules.