Mostahdaf was guided home by Jim Crowley whose legendary weighing room rival Dettori had to wait until the penultimate race of day two to enter the winner's enclosure.
The 52-year-old Italian superstar, at his farewell Royal Ascot in his final season before retirement, had endured a wretched winless opening day that also included a nine-day ban for careless riding.
But Dettori clicked with favourite Gregory in the Group Two Queen's Vase to take his overall tally of winners to 78 at the royal festival.
"I thought it was never going to come!" beamed Dettori.
"At my last Ascot to ride a winner is fantastic. I feel very sad and happy," he added.
Gregory was trained by John and Thady Gosden, who have enjoyed great success with Dettori over the years.
And the father and son team had earlier watched as Crowley eased Mostahdaf past Luxembourg and 2021 Epsom Derby champion Adayar to land the Prince of Wales's Stakes.
Both had finished in front of him in last October's Prix de l'Arc de Triomphe when he finished last.
"It was a fantastic run, John and Thady have done a great job on him," said Crowley prior to receiving his trophy from Queen Camilla.
"He was bouncing coming into the race today but I did not think he would romp home like that."
Although Gosden senior has paid many a visit to the Royal Ascot winners' enclosure, for Tom Clover it was a novel experience.
The young English trainer's Rogue Millennium gave him his first win at the meeting and the stable their first ever Group race success when she won the Duke of Cambridge Stakes.
"Our first Group win in what has been a really tough year with loads of seconds," said Clover.
"It takes some explaining and it hasn't sunk in to be honest."
Her victory under Danny Tudhope presented a conundrum for one of the members of the winning syndicate The Rogues Gallery - Rishi Persad.
He had to keep his cool as he was conducting the interviews in the winners' enclosure for broadcaster ITV.
"This is really special," said syndicate head Tony Elliott.
"I just do it for the love of racing and this is the pinnacle of racing."
Elliott's new pair of shoes bought specially for the week did not prove as resilient as Rogue Millennium - the right shoe fell to pieces.
"I will be in to the shop on Monday," he said.
'Champagne on ice'
There were two heart-warming winners in the opening couple of races.
The first produced a crackerjack of a finish as US raider Crimson Advocate just kept his nose in front from the fast-finishing Relief Rally to take the Queen Mary Stakes.
Crimson Advocate's trainer George Weaver's wife Cindy came close to death following a riding accident at Saratoga last year.
Weaver recorded the 14th-US trained winner at the meeting in what was only his second runner - his first being in 2015.
"It is a little hard for our horses coming across the Atlantic," said winning jockey John Velazquez, who was riding his fourth career winner at the meeting.
"But this meeting is so special that is why I keep coming back each year," added the three-time Kentucky Derby-winning rider.
Cindy was present but Irish trainer Jessica Harrington, who has recently finished treatment for breast cancer, stayed away.
As a result, the 76-year-old missed 25/1 outsider Villanova Queen winning the Kensington Palace Fillies Handicap.
Harrington has travelled over in a rare outing to the races since she was diagnosed last October.
However, having attended Tuesday's opening day she opted to stay behind at her close friend trainer Nicky Henderson's house - he had a winner on the opening day.
"You cannot keep her away for long, you know my mum," said her daughter Kate.
"This means a lot as it has been a very tough year.
"She will have the champagne on ice for Nicky when he gets back tonight!"
