The handsome colt made a big impression in winning April's Ballysax Stakes in the Leopardstown mud and after a deluge of rain hit the Downs earlier in the week, he was well backed to emulate his brilliant sire Sea The Stars, who won the Epsom showpiece seven years ago.
However, there was a late hindrance for his supporters when it emerged Harzand had spread a plate on the journey from Ireland, throwing his participation into doubt. The 13-2 chance was eventually given the green light, though, and was always travelling powerfully in the middle of the pack.
Smullen launched his challenge early in the straight and while 7-2 favourite US Army Ranger came storming down the middle of the track from a long way back, Harzand was always doing enough and held him off by a length and a half.
Smullen said: "It's a childhood dream to win this race and I've been at it a while now. 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.
"He's a vastly improving horse and showed his true ability today. He found another gear when the second horse came to him."
Winning owner-breeder Aga Khan was claiming the Derby for a fifth time, having previously struck gold with the likes of the legendary Shergar (1981) and most recently Sinndar (2000).
He said: "This is a major gift for everyone that works with me."
Weld has won every Classic in his homeland and numerous big prizes across the globe, but was thrilled to strike gold in the biggest Flat race of them all.
He 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 (before the race).
"But with the tremendous help of all my colleagues and my team, we made it.
"The Irish Derby will probably be his next race. I've always believed in this horse and I entered him in the Arc, so that must be a target."
US Army Ranger goes down fighting
US Army Ranger fared best of the Aidan O'Brien Investec Derby quintet, finishing second behind Harzand as Irish-trained colts swept the board at Epsom.
The 7-2 favourite was settled towards the rear of the field before rocketing home, but he was held a length and a half at bay by the winner at the finish, with his stable companion Idaho third.
O'Brien said of the runner-up: "He 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."
The Ballydoyle handler added: "Idaho ran very well in third and I think he'll come forward again, too. We'll see how we are and plan things as we go along."
John Gosden's Dante hero 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."
Magical Minding and Ryan Moore weave path to Oaks glory
Minding overcame adversity to claim her second Classic success of the year in the Investec Oaks.
Aidan O'Brien's filly was a stunning winner of the 1000 Guineas at Newmarket at the start of May and while she suffered defeat when chasing the Guineas double at the Curragh less than a fortnight ago, she was the 10-11 favourite to get back on the winning trail at Epsom.
With his mount stepping up half a mile in distance, Ryan Moore rode a patient race, but found himself in a spot of bother rounding the turn for home. For a moment it looked as though Minding would have nowhere to go, but when the gap did come, she picked up quite brilliantly and was well on top at the line.
Architecture ran an excellent race to fill the runner-up spot, a length and three-quarters away, with Harlequeen back in third.
"I just had nowhere to go, but the filly got me out of trouble," said Moore.
"She was much the best (horse). She's a very good filly."
Minding was originally set to head straight for Epsom after her Newmarket triumph, but was drafted in as a late substitute for stable companion Ballydoyle in the Irish 1,000 Guineas.
While her narrow defeat to Jet Setting at the Curragh was a big surprise, connections revealed she had suffered a minor head injury in the starting stalls but were happy to let her turn out just 12 days later. .
O'Brien said: "She's an incredible filly, what can I say? To win the Guineas the way she did, then run at the Curragh on short notice. That didn't work, but she came out of it and she was incredible.
"She has speed, class, stamina, a great mind - everything. Real heart, courage and guts had to come into it, but she had it in abundance, like all the Galileos.
"I'd say we'll probably give her an easy time but I'd imagine the lads will talk about it and see what they want to do. She's tough enough to come back here after the Curragh and soft ground.”