"This is so special for James, he has come close to winning here before but for this to happen now it is just wonderful," Mallyon said.
"The past 18 months have been tough but for this to happen just a month after his return to riding puts all the bad times aside."
McDonald has been a popular rider in the UK in recent years, with 25 previous wins including feature race successes at the Glorious Goodwood and Newmarket July Festivals in 2016.
But his 26th success in the UK was on the biggest stage of all at Royal Ascot, extending Sir Michael Stoute's record at the meeting to 77 career wins, two ahead of the late Sir Henry Cecil and 15 in front of Aidan O'Brien on 62 wins.
"It was a real honour to ride for his stable, he is a master trainer and words can't really describe him.
"He obviously trained a Group 1 winner today and he is a tremendous bloke to ride for, there is no pressure, you just go and have fun and the horses do the work."
The highly regarded Expert Eye was having his first run since failing in the 2000 Guineas behind Saxon Warrior at Newmarket in May, but he relished the drop back in grade and distance to win easily.
After settling in the back half of the field, Expert Eye produced a big turn of acceleration down the centre of the course to draw right away, defeating Society Power and long-time leader Could It Be Love.
"I was a little bit worried early on, it was a little bit interrupted and I got back a couple spots further than I anticipated but the horse has got a great turn of foot and he showed that today," McDonald said.
McDonald, who returns to Australia on Sunday, has a further eight rides booked at the meeting, including the promising Equilateral for trainer Charlie Hills in today's Group 1 Commonwealth Cup over 1200m.
- Racing.com
