British 18-year-old Raducanu, who was ranked No.150 by the WTA before the tournament and had only played in one other Major (Wimbledon earlier this year), enjoyed a sensational run at Flushing Meadows and proved too strong for Fernandez, 19, who was also contesting her first grand slam final.
Raducanu, who is the youngest women's grand slam finalist since a 17-year-old Maria Sharapova took the title at Wimbledon in 2004, showed no signs of nerves in the opening set, taking a decisive advantage.
A roller-coaster second set could have gone either way, but from a break down, Raducanu hit back to serve out the victory in an epic final between two of tennis's rising stars.
Neither player looked fazed by the magnitude of the occasion during the first set, with a series of high-quality rallies and superb winners lighting up Arthur Ashe Stadium.
Raducanu started strongly and went 2-0 up after a pulsating game on Fernandez’s serve, which lasted more than 10 minutes and had seen the Canadian save five break points before eventually succumbing.
Fernandez responded well, though, breaking back immediately before restoring parity on her own serve.
The first set went with serve until Fernandez was serving to stay in it at 5-4 down.
Raducanu squandered three set points before ultimately taking her fourth with a thumping forehand down the line, securing the lead after exactly one hour.
The British player had three break points in the second game of the second set, but Fernandez rallied to hold.
That recovery galvanised Fernandez, who broke Raducanu in the next game at the third time of asking, although her opponent broke back immediately with two wonderful backhands to see out the game.
Raducanu held her serve before opening up a 4-2 lead as Fernandez wilted under a string of excellent shots.
After a dramatic medical time out at 30-40 down on her own serve for a cut below her left knee, which left Fernandez visibly frustrated, Raducanu came back out renewed and served an ace to seal arguably the most unlikely grand slam win of all time.