Australian-trained sprinter Redkirk Warrior ($5) raced on pace but was soon under pressure at the 400m, weakening to finish in 10th position and beaten 12.75 lengths.
A son of Coolmore Stud's champion stallion Fastnet Rock, Merchant Navy began his career in Australia trained by Ciaron Maher before Aaron Purcell took over when Maher was under suspension.
It was Purcell who guided Merchant Navy to his Australian Group 1 success, winning the Coolmore Stud Stakes over 1200m at Flemington during the spring.
After that win, a deal was done by Coolmore Stud to purchase the colt for $27 million to eventually stand at stud alongside his sire. The deal allowed the previous owners to share in any additional prizemoney that the colt won despite being transferred to Irish training legend Aidan O'Brien.
O'Brien has now won four Group 1 races at Royal Ascot with ex-Australian trained gallopers, having guided Haradasun to victory in the 2008 Queen Anne Stakes, Starspangledbanner in the 2010 Diamond Jubilee Stakes, and So You Think in the 2012 Prince Of Wales's Stakes before Merchant Navy.
Winning rider Ryan Moore, victorious in his fifth race for Royal Ascot 2018 and 53rd victory at the Royal Meeting overall, believes Merchant Navy is "a remarkable horse".
"I can't repeat what I was thinking when I hit the line," Moore said, riding his second winner for the day after taking out the Group 2 Hardwicke Stakes aboard Crystal Ocean earlier in the day.
"He hasn't been up here long. He has had two starts here and won them both, beating high-class fields, and he is only a young horse.
"I sat third and I didn't really want to be third. I had to go sooner than I would have liked. The race didn't really go to script for anybody I would have thought."
Merchant Navy was purchased for $350,000 at the 2016 Inglis Australian Easter Yearling Sale from the Segenhoe Stud draft by Ciaron Maher, who was on hand to see the colt win.
"He has done it in both hemispheres and he will be very busy when he goes to stud at the end of this season," Maher said.
Tom Reilly, CEO of Aushorse, said the win of Merchant Navy was significant for the Australian breeding industry.
"Australia has got a great tradition and there has been some hard luck stories since Black Caviar came here and won six years ago," Reilly said.
"We have Merchant Navy now arguably the top sprinter in the world and Winx, the leading horse in the world.
"It is really a great time for Australian breeding."
Merchant Navy will now get his chance to emulate Starspangledbanner in the Darley July Cup at Newmarket on July 14.
- Racing.com