Dunne's second goal came just 11 seconds into the extra period, to end Canada's run of three-straight world titles as the women's under-18 championship.
Alyssa Gorecki also scored for the U.S., while Kaitlin Burt made 16 saves for the win.
Shae Labbe and Samantha Cogan supplied the offence for Canada, which earned silver for the first time since 2011. Marlene Boissonnault stopped 38 shots in net for the Canadians.
The Americans did not score on their seven power plays, while the Canadians went 0 for 2 with the man advantage.
Canada and the United States have met in all eight gold medal games at the U18 women's world championship, and each has now won four gold medals. Canada won the world title in 2010, 2012, 2013 and 2014, and settled for silver in 2008, 2009 and 2011.
Earlier Monday, the Russia beat the Czech Republic 5-1 to win the bronze medal.
Canada's Sarah Potomak was named the tournament's top forward and most valuable player. Dunne earned top defenceman honours, while Russia's Valeria Tarakanova was chosen as top goaltender.