In the first All-American final since the Williams sisters played each other for the title in 2002, Stephens ran riot to win 6-3 6-0 in a surprisingly one-sided encounter on Arthur Ashe Stadium.
Stephens joins 2009 champion Kim Clijsters as the only unseeded women's champions in the Open era.
"It's incredible," said Stephens. "If someone told me when I had surgery that I would win the US Open, it's impossible I would say. Coming back, just being able to keep it all together, this journey's been incredible and I wouldn't change it for the world.