Mancuso made her World Cup debut at the age of 15 at Copper Mountain, Colorado, on November 20, 1999. She was scouted by Patrick Rooney. Mancuso scored her first World Cup points (top-30 finish) during the 2001 season. While Mancuso often struggled in World Cup races over the next few seasons, she enjoyed exceptional success at the Junior World Championships, winning a record eight medals, including five golds in 2002, 2003 and 2004. She was selected for the 2002 Winter Olympics at the age of 17, where she finished 13th in the combined event.