The jockey enjoyed an emotional and proud success earlier on the card when Desert Hero landed the King George V Stakes for His Majesty The King & Her Majesty The Queen.
Ed Walker's colt came with a well-timed challenge inside the final furlong having been switched off heels and the son of Australia displayed a willing attitude to prevail by a comfortable two-and-a-quarter lengths from Exoplanet (12/1).
The Charlie Appleby-trained Bold Act (9/1) finished back in third with the 4/1 favourite, Torito, taking the fourth spot for John and Thady Gosden.
Walker said: "The worry was the close proximity to the Derby, only 19 days, as not many horses back up from the Derby to Ascot successfully. We haven't done a lot with him at home, but he's seemed well. If there was a brilliant alternative in a couple of weeks' time, we may have been tempted, but there wasn't and this was the perfect race for him.
"I have quite a long-standing team now at home and Charles Guet, my head lad who has been with me for 10 years, rides him every day and he was happy with him. He was well so we rolled the dice and it paid off.
"It's been an agonising wait between drinks [Royal Ascot winners] and we've had quite a lot of seconds. Last year we had second in the Kensington Palace, the Wokingham, and we were second yesterday in the Duke of Cambridge. It has been a bit frustrating, so I am really pleased to have kicked that one home.
"It's a great day for Tom. I was so pleased to see him have that winner for His Majesty, that was very important for all of us. Tom has been a big part of our team now for a number of years, he rode our first Group One winner Starman and it's great to get another one on the big stage."
Walker said that it "is without doubt the hardest place in the world" to have winners, adding: "My great friend Tom Morley brought a filly [Cynane] over for the Queen Mary and I said, 'Tommy, listen, you're not going to win – just enjoy the ride, because no-one wins here, it's so hard to win here!' So it's such a big achievement and it really is tough. We realise now the enormity of the challenge and it feels even better."
Tom Marquand said: "I planned on being a lot closer to the pace. In the Derby, I got caught too far back. It was the way the race was run and I got off thinking 'he's a good horse'. He just didn't stay, it was as simple as that in my mind, and I know Ed thought so too.
"Ed retained faith in him and, when I offered to go in and sit on him the other morning, just to see how he was, he said 'no, I'll keep him fresh thanks'.
"The race materialised well for us. There was a nice tempo and, as ever with that bend, you can sit and wait or swing wide, and we decided to swing out and go for a clean run. Fortunately for me I was lucky. I got a couple of the splits that I needed to get that run and he showed an exceptional turn of foot to burst through as he did and put the race to bed in a matter of a few strides.
"You'd like to think he is still an improver and I think today solidified what we thought after the Derby. We were obviously disappointed we didn't run closer that day but we were really pleased with the run, and he just didn't stay a mile and a half, it's as simple as that. Back to a mile and a quarter today he showed his dominance, and hopefully he can now go on a path and show his stamp as a top-class horse.
"It's huge to ride a Royal Ascot winner for Ed. As much as I love riding for him as a trainer, he's been great to me. I go in there for a coffee and we have a chat. He has been a great mate and I'm delighted to repay him with a Royal Ascot winner on such an important occasion.
"It has been a magical day."
Exoplanet's rider David Egan said: "We got a nice, smooth passage from a wide draw, had cover, relaxed well. He got a little bit of a bump in the straight. He's a very classy individual, but take nothing away from the winner, he was good."
