The Canadian, who turned 19 on Monday, stunned world number two Aryna Sabalenka in a dramatic three-set semi-final.
She will face Britain's Emma Raducanu in the first US Open final to feature two teenagers since 17-year-old Serena Williams beat 18-year-old Martina Hingis in 1999 – before this year's finalists were born.
Fernandez, ranked 73 in the world, has laid waste to a who's who of women's tennis this fortnight.
Fernandez had the crowd firmly behind her and the roar inside Arthur Ashe Stadium as Sabalenka sent the final point long could probably be heard in her hometown in Quebec, a neighbour of New York State.
She said: "I think I've been doing some incredible things. I don't know. It's like I think one word that really stuck to me is 'magical' because not only is my run really good but also the way I'm playing right now.
"I'm just having fun, I'm trying to produce something for the crowd to enjoy. I'm glad that whatever I'm doing on court, the fans are loving it and I'm loving it, too. We'll say it's magical."
Second seed Sabalenka came roaring out of the blocks, playing some near-perfect power tennis as she hit nine winners and no unforced errors to lead 3-0.
But two double faults from the Belarusian helped Fernandez break back and the youngster saved a set point as she clung on for a tie-break.
With errors creeping in from both players it was Sabalenka who tightened up the most, a horrid overhead miss and another double fault allowing Fernandez to snatch the opening set.
It was a similar story at the start of the second as Sabalenka broke to love but Fernandez, eating up her opponent's increasingly frequent second serve, levelled for 2-2.
Sabalenka took her anger out on her racquet during the changeover, earning herself a warning from the umpire.
But, frustration vented, the Wimbledon semi-finalist secured another break for 5-4 and served out to love to take the match to a deciding set.
The pair exchanged breaks midway through the third but could not be separated until Sabalenka wilted on serve, and Fernandez sank to her knees after completing another memorable victory on her remarkable journey to a first grand slam final.
Sabalenka said: "It seems like in these two weeks everything is working well for her. Like this is nothing to lose.
"She's staying on the baseline, hitting I would say sometimes crazy shots, and everything is going in. I would say everything is going her way."