The former US Open finalist maintained her stunning form throughout the week as she beat Jasmine Paolini 6-2 6-3 on Sunday (Monday AEDT).
After the 21-year-old converted match point the Canadian team celebrated wildly on court in front of their jubilant fans.
"I don't know what to say," Canada's captain Heidi El Tabakh said. "I'm so proud of this team, these girls are incredible.
"It's a dream come true."
The victory meant Canada now hold the two global tennis team event titles, after their men won the Davis Cup, also in Spain, in 2022.
Teenager Marina Stakusic had earlier put Canada within sight of glory thanks to a stunning victory over Martina Trevisan.
The 18-year-old Stakusic, 258th in the WTA rankings, produced a nerveless display to win 7-5 6-3 against an opponent ranked 215 places higher at No.43.
Only when victory was within her grasp did Stakusic wobble when four match points went adrift as Trevisan served at 2-5.
But she shrugged off that disappointment and powered through her next service game, ending the contest with a crunching forehand winner.
"I'm so happy and honoured that I could play this week, this has been the best week of my life," Stakusic, who did not even play a WTA main draw match this year, said on court.
It was then over to Fernandez who duly claimed her fourth singles win of the week by outclassing Paolini.
Fernandez had produced heroics the previous day against the Czech Republic when she beat Wimbledon champion Marketa Vondrousova to keep Canada in the semi-final.
She then teamed up with Gabriela Dabrowski to beat Czech multiple grand slam doubles champions Barbora Krejcikova and Katerina Siniakova.
Canada's team will receive $US2.4 million ($A3.8 million) in prize money -- a record for the team event formerly known as the Fed Cup and often called the World Cup of women's tennis.
They were also presented with the trophy while team members received 'Billie Blue' jackets like the ones so-loved by Billie Jean King, who watched the final from the sidelines.