Raducanu, ranked 150th, was the first British woman in 44 years to win a Slam crown after dispatching 73rd-ranked left-hander Fernandez 6-4, 6-3.
Not since Virginia Wade in 1977 at Wimbledon had a British woman taken a Slam singles title, with Raducanu the first Briton to claim the US Open crown since Wade in 1968.
Wade and British men's tennis legend Tim Henman were among a sellout crowd of 23,700 that created an electric atmosphere at Arthur Ashe Stadium in the first-ever women's Slam final between unseeded players.
The showdown of prodigy talents was the first all-teen Slam final since 17-year-old Serena Williams beat 18-year-old Martina Hingis for the 1999 US Open crown.
Raducanu is the youngest US Open champion since Williams in 1999 and the first US Open women's champion not to drop a set since Williams in 2014.
Raducanu had the fewest Slam starts of any women's Slam winner, reaching the fourth round in July at Wimbledon in her only prior Slam appearance while Fernandez had won only four matches in six prior Slam appearances.