Dermot Weld landed the Investec Derby for the first time as Harzand overcame early-morning drama to win the Epsom Classic at 13-2.
Winner of the Ballysax Stakes at Leopardstown in April, the son of Sea The Stars hit the front with over a furlong to run and although 7-2 favourite US Army Ranger finished powerfully, Harzand was always doing enough and held him off by a length and a half.
Idaho (14-1) was best of the rest in third.
Harzand had suffered a scare on Saturday morning after spreading a plate and Smullen hailed the team at the Weld yard for their efforts in getting him to the start line.
He said: "It's unbelievable. I'm delighted to have ridden the winner for the boss (Weld).
"The trouble we had with him this morning, I didn't know if he would get here, but it's a great team effort. I'm delighted for them.
"To be fair to this horse, he's a very good horse and he's improved out of all recognition."
A promising fifth on his only juvenile appearance at Gowran Park last autumn, Harzand bolted up on his seasonal return at Cork before his Ballysax triumph in the Leopardstown mud.
He was heavily backed to claim Derby glory after the rain that fell on Epsom during the week and was always travelling strongly in the middle of the pack.
The pacesetting Port Douglas led the field into the straight, but soon faltered and French raider Cloth Of Stars and 1000 Guineas runner-up Massaat were among those ready to challenge for the lead.
But Harzand picked up best, mowing down Idaho and galloping clear for a thrilling success.
Winning owner-breeder Aga Khan, who has previously tasted Derby glory with the likes of the legendary Shergar (1981) and most recently Sinndar (2000), said: "Obviously this is a major gift for everyone that works with me."
Weld said: "I won my first Classic here with Blue Wind (1981 Oaks) many moons ago, when I probably didn't have horses like this to win this great race.
"It's wonderful to do it. We had a huge worry this morning as he spread a plate travelling over and he was a very doubtful runner right up to about an hour ago.
"But with the tremendous help of all my colleagues and my team, we made it."
Aidan O'Brien was responsible for both the runner-up US Army Ranger and the third home Idaho.
He said: "He (US Army Ranger) ran a great race and I'm delighted for Dermot Weld and the Aga Khan.
"He was a bit babyish early on, but we're not making any excuses.
"We'll see how the horse is and we're looking forward to running him again.
"Idaho ran very well in third and I think he'll come forward again, too."
John Gosden's Dante winner Wings Of Desire was best of the British in fourth.
Gosden said: "The horse has done us proud. He's done nothing wrong.
"It was a bit rough out the back, but he got a nice run down the straight, but it was not his ground.
"He will be better on better ground, but he's learning a lot.
"The first three really appreciated the ground and the winner was very smart.
"We'll look towards the Curragh (Irish Derby), if it dried up, but we won't be going to Ascot for the King Edward, which was our original plan."