Leopold started his professional career in the NHL in 2002. He played 58 games for the Calgary Flames and scored four goals and 14 points. In his second season, Leopold would play in all 82 regular season games, scoring 33 points as well as contributing to the Flames' playoff run to the 2004 Stanley Cup Final, play which earned him a spot in the defense of the American national team for the 2005 World Championships as well as for the 2006 Winter Olympics.