It was a stunning achievement for the unheralded 150th-ranked teen, who didn't drop a set in three qualifying matches and seven main draw matches over two weeks on the New York hardcourts in producing a stunner for the ages.
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.
Queen Elizabeth II was among the first to pay tribute to Raducanu's remarkable win.
"It is a remarkable achievement at such a young age, and is testament to your hard work and dedication," the British monarch said in a statement.
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.
"It means so much to have Virginia here and also Tim and to have such British legends and icons, to follow in their footsteps and it gave me the belief I could actually do it."
Fernandez, who turned 19 on Monday, had ousted defending champion Naomi Osaka, second-ranked Aryna Sabalenka, fifth seed Elina Svitolina and three-time Slam winner Angelique Kerber in her epic run to the final.
"I'm very proud of myself the way I played the last two weeks," Fernandez said.
"I hope to be back here in the finals and having the right trophy," added a tearful Fernandez, before paying tribute to New York as the city marked the 20th anniversary of the 9/11 attacks.
"I know it's especially hard on this day for New York," she said. "I just want to say I hope I can be as strong and resilient as New York has been the past 20 years."
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.
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.
"Both of us were playing fearless tennis for two weeks," Raducanu said. "I hope we play each other in many more tournaments and hopefully finals.
"It shows the future of women's tennis and the depth of the game right now is so great."
Raducanu has 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.
Fernandez saved five break points in her opening service game of the match before finally surrendering the 10-minute game on a netted forehand to give Raducanu a 2-0 edge.
Fernandez broke back in the third game, Raducanu netting a backhand on the fourth break point she faced.
But Raducanu broke in the 10th game to take the set, blasting a forehand winner to seize the lead after 58 minutes.
In the second set, Fernandez broke to 2-1 but Raducanu blasted a backhand cross-court service return winner to equalize 2-2 and later whipped a forehand passing winner by the Canadian to break for a 4-2 advantage.
Raducanu had two match points on Fernandez's serve in the eighth game but sent a backhand wide and netted a forehand as Fernandez held to 5-3.
Fernandez grabbed a break chance at 30-40 in the ninth game when Raducanu skidded on the court on her left knee and opened a bleeding cut, the match halted while a trainer bandaged the wound.
When play resumed, Fernandez sent a forehand long to squander the break chance and Raducanu ended matters after an hour and 51 minutes on her third ace of the match.
"I was going for that serve the whole match. I can't believe I finally hit it," Raducanu said. "I was just praying not to double fault."