The result, a repeat of last year, was met with raucous cheers from the bumper crowd that had crammed onto the grandstand to witness one of racing's greatest duos.
The cheers continued into the parade ring with Dettori whipping the crowd into a frenzy with his famous flying dismount.
The Italian jockey has now ridden seven Gold Cup winners, only Lester Piggott has more with 11.
"There's never been a jockey like him to work the crowd. He's infectious. You just knew he was going to do it," veteran racing pundit Brough Scott told ITV.
"This is a day of sporting history, four winners on Gold Cup Day, culminating in the Gold Cup."
Stradivarius, owned by Bjorn Nielsen, was the hot favorite after last year's victory and started the race with odds of even money.
With the crowd on tenterhooks, Dettori positioned his horse perfectly and the way he stormed down the home straight to edge out Dee Ex Bee caused hairs to stand up on the back of necks.
"What an amazing horse he is, he's a horse for the big occasion and I love him dearly," Dettori told ITV after the race. "The people love him."
Stradivarius is now on a seven-race winning streak, with his last loss coming here at Ascot in October 2018.
The victory gave Dettori his fourth win out of four on day three and recalled memories of his "Magnificent Seven" when he won all seven races on the card at Ascot in September 1996.
"It brings back memories of 1996," he told ITV after his third win on Star Catcher.
With Dettori on a winning streak, the odds for his fifth race of the day dramatically dropped from 14/1 to 7/2 favorite.
The atmosphere trackside reached fever pitch as the horses broke from the gate, with almost every punter having put their faith in Dettori and his ride Turgenev.
As the Italian broke from the group to lead into the final furlong, the place erupted with excitement.
Despite the cacophony of noise urging him to the line, Dettori was ultimately pipped at the line by outsider Biometric.
"20 years ago I would have won that," joked Dettori afterward.
Champion trainer Gosden was full of praise for a jockey who seems to only get better with age.
"Frankie [Dettori] is like a player who gets to the final of Wimbledon and then raises it to another level," Gosden told reporters.
"He is a phenomenon. For a kid of 48 he is remarkable and when he rides here he goes to another level."
It's been an impressive festival for Dettori, who has now registered 66 career wins at Royal Ascot.
"I embrace the pressure. It is part and parcel of the job and I thrive off it," he told reporters earlier Thursday.
His success on the track, along with his loveable persona off it, makes Dettori one of the most famous jockeys in the world and a certified fan favorite.
He routinely draws in the crowds wherever he goes and transcends the sport of horse racing.
"You need strong characters and you need strong figureheads. He's the face of the sport," said racing fan Andrew Fisher.
"You need people like him to make it interesting really, you connect with people like them."
Over the course of his glittering career, Dettori has ridden some of the best horses on the planet. He was Godolphin's retained jockey for 18 years -- riding 110 Group 1 winners for stables of Dubai ruler Sheikh Mohammed.
He went freelance in 2013, and has recently combined with Briton Gosden with spectacular success. Among the highlights in 2018 was a second straight victory in the Prix de l'Arc de Triomphe and a Breeders' Cup Turf title, both on board wonder horse Enable.
The jockey is now a household name, a rarity for members of the horse racing world.
"I don't really come to racing very often but if you ask me to name one jockey, he's the only one I know," said spectator Leslie Paine.
The Gold Cup may not be the richest race during Royal Ascot week but it is certainly the most prestigious.
Founded in 1807, the Group 1 race is also the oldest at the festival and is the ultimate test for some of the best long-distance Flat racers in the world.
Run over a marathon trip of two-and-a-half miles, many champion horses have distinguished themselves on this very special day of racing.
Perhaps what makes it so special is the presence of the Queen, who always presents the trophies -- not to mention the prize of $635,000 (£500,000) lining the winning connections' pockets.
Royal Ascot was gripped by Frankie fever Thursday.
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